Ivalice is the world which comprises the settings of FF Tactics, FFTA, FF12, and Vagrant Story. Although the connections are tenuous, there is much evidence that the four games really are meant to take place in the same world. An absolute timeline is difficult to construct, so for the moment we would like to at least present the common themes throughout the different games. Note that FFTA's Ivalice is actually a "fictional" Ivalice, brought to life from a book called the Gran Grimoire, and based on a video game called "Final Fantasy". Hence, it's almost certain that it is not directly connected with the other games.
The twelve signs of the Zodiac play an enormous role in Ivalice. In FF Tactics, every character has a sign which affects the damage ratio to units with similar or opposite signs. The story revolves around the Zodiac Stones, which are intimately connected with the demon Lucavi. The story itself is subtitled "The Zodiac Brave Story". It also introduces Serpentarius, the thirteenth Zodiac sign. FF12 continues the theme with Zodiac gems sprinkled around the world as general loot items, the best spear and shield being called the Zodiac Spear and Zodiac Escutcheon, twelve Zodiac gates in the Great Crystal area, and thirteen summonable Espers, each representing one sign. Many of the Espers are former Lucavi from FFT, others are Totema from FFTA; bosses from FF1, 4 and 5 and the Zodiac summon from FFT also are involved. Most of them have some sort of connection with their Zodiac sign. Here is a complete list.
      | Belias
 FF12 Title: The Gigas
 Also Appears In: FFT as Velius the Warlock
 Sign: Aries, the Ram
 Limit Break: Hellfire (Ifrit's usual spell)
 Element: Fire
  CP: Mesha Ascendant. Scion of darkness and guardian of the Holy Realm, made by the gods in opposition to the Transcendent Loghrif, scion of light. Called the Gigas for his appearance: man and monster fused as one. Considered a mistake upon his making, and receiving not his intended role, the Gigas challenged the gods and lost. Scorned by his masters, he found another: the Dynast-King, whose tomb he swore to protect for eternity.
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        | Mateus
 FF12 Title: The Corrupt
 Also Appears In: FFTA as the Human Totema.
 Sign: Pisces, the Fish
 Limit Break: Frostwave
 Element: Ice
  CP: Meena Ascendant. Scion of darkness ruling and protecting those who live in the underworld, in opposition to Lahabrea, Abyssal Celebrant and scion of light. In the course of his rule, he submitted to avarice, and the darkness took his heart, transforming him until he was both evil and corrupt. Then in his cowardice did he bind a Goddess of the Demesne of Ice, and using her as a living shield, he challenged the gods. Defeated before their might, he fell screaming into the depths of hell, there to be imprisoned for eternity.
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  | Shemhazai
 FF12 Title: The Whisperer
 Also Appears In: N/A
 Sign: Sagittarius, the Archer
 Limit Break: Soul Purge
 Element: Soul
  CP: Dhanus Ascendant. Scion that is both horse and woman, wielding utter control over the souls that wander the underworld, in opposition to the Martyr Igeyorhm, scion of light. Though she once served the gods as a guardian, when Ultima announced her rebellion, Shemhazai went to her, whispering of the gods' hidden weaknesses. She then descended upon the land without leave of the gods, and taught men of destruction and evil. For this was she stricken down and bound.
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      | Hashmal
 FF12 Title: Bringer of Order
 Also Appears In: FFT as Hashmalum the Regulator
 Sign: Leo, the Lion
 Limit Break: Gaia's Wrath (Titan's usual spell)
 Element: Earth
  CP: Simha Ascendant. Scion sent by the gods to wield and manipulate the laws of his world, and with holy power lead mankind to order. Created in opposition to Fandaniel the Protector, scion of light. Desiring to bring order to all things, he joined with Ultima in her battle against the gods. He gave his body to the Thousand-Years War, and when his strength was spent, down into the burning inferno he fell.
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        | Famfrit
 FF12 Title: The Darkening Cloud
 Also Appears In: FFTA as the Moogle Totema.
 Sign: Aquarius, the Water Carrier
 Limit Break: Tsunami (Leviathan's usual spell)
 Element: Water
  CP: Kumbha Ascendant. The hideous, darkly clouded form of Famfrit, scion in opposition to Holy Queen Emmerololth, scion of light, was anathema even to his creators. Thus, after a great battle, was he broken and sealed within armor laced with wards. The confines of his armor are void of light, and so is he called the Darkening Cloud. Men fear the rain that falls from the black clouds that ooze from that giant ewer as a herald of chaos and waste.
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            | Adrammelech
 FF12 Title: The Wroth
 Also Appears In: FFT as Adramelk the Ghost of Fury; in FFTA as the Bangaa Totema.
 Sign: Capricorn, the Goat
 Limit Break: Judgment Bolt (Ramuh's usual spell)
 Element: Thunder
  CP: Makara Ascendant. Emperor among the scions, able to reduce to nothing aught he strikes with a single vengeful blow of his fist, created in opposition to Deudalephon the Benevolent, scion of light. Though he was made by the gods to quell the fiends that raged in the Otherworld, his immense strength and fearsome visage drew the fiends to his side, and turned him against his creators. Adrammelech rose to prominence in the Otherworld, whence he led a fiendish horde against the gods, but in the end, he was defeated.
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      | Zalera
 FF12 Title: The Death Seraph
 Also Appears In: FFT as Zarela the Dark Angel
 Sign: Gemini, the Twins
 Limit Break: Condemnation
 Element: Death
  CP: Mithuna Ascendant. Heretic scion who wrapped the world in dark energies, seeking to take the souls of all living things to himself. Created in opposition to Emet-Selch, Angel of Truth, and scion of light. Originally tasks with the judging of men upon their deaths, his soul was tainted by the curses of those who raged against the heavens, and seizing one of the gods' servants, a shamaness, as a hostage, he rebelled against his creators. Even now, in defeat, he clutches the shamaness to him in his right arm, and with the aid of her death-wail does he summon the soul of darkness to do his bidding.
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                  | Exodus
 FF12 Title: The Judge-Sal
 Also Appears In: FF5 as Exdeath, the final boss; in FFTA as the Viera Totema.
 Sign: Libra, the Scales
 Limit Break: Meteor (used in FF5 as well)
 Element: None
  Note that his FFTA version had a tree theme, which is closer to Neo Exdeath, whereas his FF12 version is closer to Exdeath's regular form. The backstory presented in the Clan Primer coincides with FF5.
  CP: Tula Ascendant. Most ancient of the scions, created in opposition to Halmarut, the Arbiter, and scion of light. Tasked with keeping watch over the world, with the authority to judge the value of all things. As he watched, unseen, unknown, his attachment to the world dwindled and faded until it was as nothing. Fitting that he would desire to make the world, too, as nothing. Yet he fell in the war against the gods, and was thwarted, imprisoned in punishment for his heresy.
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      | Cúchulainn
 FF12 Title: The Impure
 Also Appears In: FFT as Queklain, the Impure King
 Sign: Scorpio, the Scorpion
 Limit Break: Blight
 Element: Poison
  CP: Vrishchika Ascendant. Scion created to rid the world of its impurities by swallowing them within himself, in opposition to Nabriales the Majestic, scion of light. The world, however, was more filled with impurity and corruption than even the gods dared imagine, and having swallowed it all, the once beautiful Cuchullain was transformed into a hideous thing, a deity of filth, and so did he turn against his creators. Wherever his feet should fall, there all life withers to dust.
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      | Zeromus
 FF12 Title: The Condemner
 Also Appears In: FF4 as the final boss
 Sign: Cancer, the Crab
 Limit Break: Big Bang (used in FF4 as well)
 Element: Gravity
  CP: Karka Ascendant. Honoring the law more than any other, a scion of holy order and condemner of criminals. Created in opposition to Knight-Star Pashtarot, scion of light. He turns his deep, abiding hatred for those who break the law into living darkness, therein to plunge the guilty in fell judgment. Over time, he came to care less for upholding the law and more for condemnation, and so tainted by hate, he sought to condemn the gods themselves to death. Thus did he earn the title of "The Condemner", and thus did he fall from grace.
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            | Chaos
 FF12 Title: Walker of the Wheel
 Also Appears In: FF1 as the final boss; in FF7 DoC; in FFU.
 Sign: Taurus, the Bull
 Limit Break: Hurricane
 Element: Wind
  Note that the Clan Primer backstory coincides with FF1's story.
  CP: Vrishabha Ascendant. Tutelary deity of the sacred crystals fasioned by the gods at the time of the Great Making. Created in opposition to Mitron the Chastiser, scion of light. Upon entering the world of Man, he was enveloped in the turmoil rampant there. Lost, he died and was reborn countless times, a walker of life's wheel, eventually to rage against the gods that had so fated him. By sitting in meditation upon the Uneh Pedestal does he clear heart and mind until all that has order and reason and thought is made as nothing.
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        | Ultima
 FF12 Title: The High Seraph
 Also Appears In: FFT as Altima, the Arch Angel and final boss; in FFTA as the Nu Mou Totema.
 Sign: Virgo, the Maiden
 Limit Break: Perfect Ultima
 Element: Holy
  CP: Kanya Ascendant. Masterpiece among the scions created by the gods, and mastermind of the plot to rise against them. Prior to her betrayal, she was tasked with guiding souls to heaven and aiding in their reincarnation. Called the High Seraph for her angelic wings of glimmering gold, yet it was on wings of deepest black that the tainted angel Ultima rose against the gods. Since her fall, her heart is without light, and impossible to know.
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    | Zodiark
 FF12 Title: Keeper of Precepts
 Also Appears In: FFT as Zodiac, the final summon
 Sign: Serpentarius, the Snake
 Element: Dark
 Limit Break: Last Eclipse
  CP: Ashlesha Ascendant. Strongest of the scions created by the gods, they feared his growth, and so kept him a child. So indomitable is his strength that all things are by him twisted and pressed into oblivion. He alone fasions the laws governing all things, and administers punishment in place of the gods. So is he Keeper of Precepts, and his authority is absolute.
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There are several crossovers between the games. Again, this is not necessarily a proof that they are factually connected (the names may just be cute references).
It is difficult to completely reconcile all areas of geography in the various games, but the names of many of the countries, continents and areas do cross over fairly consistently.
There are several places which exist in both FFTA and FF12; however, since FFTA's places are arbitrarily positioned and often have no direct bearing on the story, there's no indication if they're really meant to be the same. These place names are: Eluut Sands (possibly turned into Eruyt Village in FF12), Giza Plains, Ozmonfield/Ozmone Plain, and the Salikawood. Al-Cid also mentions the "Ambervale of Clan Margrace" in Rozarria; in FFTA, Ambervale was also a private retreat for royalty.
Lesalia - It's possible that Lesalia from FFT is identical with Rozarria in FF12. Not entirely sure, though.