If you want to read about the civilisations in detail, see the drop down menu to the top right of this page, but here is a brief overview.
All text below this is out of date, for civ details see each individual civ page.
Afghans
The Afghan's civilisation bonuses and tech tree synergise to provide a lot of counter options, and to aid in transitioning between various army compositions. Their unique unit, the Khorasani archer, is a bit out of the box, with an attack bonus to help against infantry with high pierce armour, and some armour to help against skirmishers, they might struggle a little against heavy cavalry and siege. The Afghans have a lot of flexibility in their tech tree, but may struggle to win through brute force as they lack several key power units.
Bantu
The Bantu are almost like a mesoamerican civilisation, without access to gunpowder or much cavalry. Their civilisation bonuses and tech tree reflect their strengths with foot units and siege. Their unique unit, the Assegai spearman, is a well rounded infantry unit, which is fast with decent pierce armour and an attack bonus vs cavalry, but might be a little vulnerable to other stronger infantry. The Bantu will try to use a balanced army composition of infantry, archers, siege and possibly elephants to dominate in a frontal assault.
Caribs
The Caribs excel at sneak attacks, and are one of the better rushing civilisations on water maps. They have two unique units; the first is the Canoe which is a cheap scouting ship that can provide the Caribs with valuable map information and allow their initial attacks to be as damaging as possible; the second is the Blowgunner, which behaves much like an unmounted Arambai but is also has the unique ability in that it is invisible on the minimap, like the Arambai it is vulnerable to other ranged units with superior accuracy. Lacking the diverse array of power units available to other civilisations, the Caribs will need to rely on their speed and flexibility early in the game to put them in a good position later on.
Chola
The Chola have no interest in subtlety, they love to use tough infantry, demolition ships and elephants to flatten their opposition. The Chola also have two unique units; the first is the Katarkollai which is a cheap spammable warship; the second is the Bombard elephant, which is exactly what it says on the tin, able to deal and take an extraordinary amount of damage, but watch out for monks, camels and pikemen. Whether it be on the land or the water, the Chola have options to make things exciting.
Dutch
The Dutch are all about gunpowder and navy. Their unique unit is the Musketeer, which is a hand cannoneer variant with an attack bonus vs cavalry, but is vulnerable to other ranged units and siege weapons. The Dutch would love nothing more than to gain water control with their powerful warships, turtle behind their fortified walls and boom up to the imperial age and then cover the map with Bombard towers protected by Bombard cannons and Musketeers.
Georgians
The Georgians are another civilisation which likes to take its time in conquering its enemies, either with monks or heavy cavalry. Their unique unit is the Monaspa, which is a heavy cavalry unit and unique amongst melee units in that it does pierce damage, this means that although they trade very favourably vs most units with low pierce armour, they are for cavalry unusually ineffective against buildings, skirmishers and siege. The Georgians have excellent cavalry options, with faster researching paladins, a cost efficient unique unit, hussar and even the options of camels; but you might have to get creative to protect them from pikemen.
Helvetians
The Helvetians take the philosophy of war to the next level, and have put all their eggs into one basket: the pike and shot, counter that strategy and you will go a long way in defeating them, but good luck with that. Their unique unit, the Swiss pikeman, takes the strengths of the champion and halberdier and puts them into one unit, although they remain vulnerable to archers. The Helvetians do have other options, with good archers, siege, monks and defences but are slow moving on the account of their abysmal cavalry.
Iroquois
The Iroquois don't like to complicate things: make siege, protect siege with infantry, win! Their unique unit, the Mantlet, is a siege archer which functions a bit like the mesoamerican version of the Bombard cannon, able to perform against both buildings and units, but as they have a minimum range watch out for melee units. And if that isn't enough, they have good archers and eagle warriors that are dirt cheap and just will not die.
Kanembu
The Kanembu like to be a little unconventional, but with options to counter all units and have some power options of their own. They have two unique units; the first the Lancer is a trash camel available to the Kanembu and all their allies; the second the Mafarauci is an extraordinarily cheap archer which complements their cavalry options well but is very vulnerable to skirmishers and siege. The Kanembu have the most well rounded stable of any civilisation, with fully upgraded Lancers, Light cav, Cavalier, Heavy camels and Battle elephants. If that's not enough, their accurate scorpions and enlightened monks are intriguing options.
Khazars
The Khazars are an interesting civilisation, with a range of flexible cavalry archer, tower and siege options allowing them to perform well on both open and closed maps. They have a unique building, the Ballista tower, which functions much like a scorpion although it has much more durability and zero mobility; and an unique unit the Arsiyah, which is an anti-cavalry cavalry archer, which may prove a poor choice if used against unmounted opponents. Their biggest weaknesses are their infantry and navy, if you can combat their powerful anti-cavalry cavalry options or fight them on the water they may struggle.
Mississippians
The Mississippians have no interest in playing a beautiful meta-game, instead they prefer to make a mess of proceedings with forward fortifications to take map control. They have a unique building, the Fortress, which is a weaker version of the Castle and can produce all barracks units, and two unique units. The first, the Mobile palisade available to all Mississippian allies, is simply a giant wooden shield that can be moved around the battlefield to absorb arrowfire; the second, the Falcon warrior, is in many ways an improved Champion, with the same strengths and weaknesses. If you can create a strategy to halt the forward crawl of their infantry and fortification deathball, you will go a long way towards defeating the Mississippians.
Nepalese
The Nepalese are the masters of the choke point, and have a diverse array of options to prevent passage of any form of unit. The Nepalese have two unique units: the first, the Imperial scorpion, is available to all Nepalese allies who also have access to the Heavy scorpion which it is an upgrade of; the second the Yak rider, has the most hitpoints of any non-elephant cavalry unit and is a very cost effective meat shield, but remains vulnerable to pikemen. When playing as the Nepalese, remember to ensure you are always fighting on your terms, and to think outside the box if you encounter Bombard cannons.
Nubians
The Nubians do two things well, archers and camels. Their unique unit, the Archer of the eyes, adds to those strengths, having improved attack, range and rate of fire compared to a standard archer, but with the same vulnerabilities. The Nubians excel in open engagements where they can harness their archers and camels to take favourable engagements, avoiding being pinned down is key to victory.
Polynesians
The Polynesians love to expand their empire and sprawl all over the place, get in their way and you might find their patience thin. Their unique unit is the Maori warrior which is key to this philosophy, with excellent piece armour and large attack bonuses against civilians and buildings, but is vulnerable to heavy infantry and cavalry. Access to the water is key to the Polynesian's expansion, without it you must harness their other strengths very deftly to achieve victory.
Tartars
The Tartars are all about military dominance, as such they will fall behind other civilisations if you do not make, upgrade and use your military units. Their unique unit is the Catapult, which is a Onager/Bombard cannon hybrid which has a particularly fast projectile speed, making it excellent at bypassing micromanagement of its targets, although it shares the vulnerabilities of most siege units. The Tartars have very strong options from their archery range, protect them from skirmishers and siege with the similarly strong Tartar cavalry and siege and you can create a very flexible and powerful army.
Thai
The Thai might start a little slowly, but once they have enough economy to sustain cavalry or elephant production they can overwhelm their opponents with both quantity and quality. Their unique unit is the Krabi warrior, which functions much like a Champion with the added trait of being an excellent counter to monks. Speaking of monks, the Thai monks are some of the best, along with their allies they regain faith faster than any others. With a poor archery range, the Thai may need to get creative to protect their cavalry from pikemen.