There are 3 distinct civilizations in AoM to choose from, with each offering the player the choice of 3 major gods to choose from at the start of a game. There are 4 ages in a standard game (less if you win or lose early!) consisting of the Archaic age, the Classical age, the Heroic Age and finally the Mythic Age. Each time the player begins research for the next age, they will be offered a choice of two minor gods to worship and gain benefits from. Choose carefully though as once the research is completed, you will not be able to rescind the choice made. This means that for a full game, the player will end up with the major god chosen at the start, as well as three minor gods, each more powerful than the last.
The civilizations which can be chosen are the Greeks, the Egyptians and my personal favorite, the Norse. Each gains favor (a resource needed to build all mythological units and research) in a different way. Each also plays differently.
Greeks
The Greeks are possibly the easiest civilization to play in AoM because they play almost exactly like any civilization from earlier Age of games. Greeks gain favor by worshipping at the temple they build in the Archaic age. It is important to note that favor is gained by the Greeks in a diminishing returns system. Greek heroes are all named units from stories everyone knows. They are very powerful units and as a result are capped at a limit of 4 heroes. They are researched at the town center and each major god grants unique heroes at each age advance. The only exception to this rule is Poseidon, who grants an extra hero who is researched at the dock. This unique hero is The Argo and is a naval only unit. All other Greek heroes are land only. Greeks start each game with a scout unit called a Kataskopos. He can not be replaced if killed, although he is fairly swift so should be able to scout large amounts of the map before he is killed. Once a temple is built, the Greek player can also build Pegasus as much as they like. Pegasus is an aerial unit and as such can only be killed by ranged units or buildings. Each Greek major god also grants a unique unit in the Mythic age, built at the fortress.
Egyptians
Egyptians are slightly more difficult to play, mainly because one of their best units (the Pharaoh) is a unit unlike any unit in any other game. He is given the ability to 'empower' buildings. This simply means he is able to speed up yuoa ayret pa lej fh fyfarao ris ythe maopstant impritenet og reay youur units
Norse
The Norse are the Vikings of old. They are perhaps the most difficult to get used to because of the way they build and gain favor. Whereas the Greeks and Egyptians use their laborers to build all their buildings, the Norse use their infantry. This makes it very easy to forward build or rip down an enemy Town Center and replace it with your own very quickly. Norse soldiers revel in combat, and this is reflected in the way the Norse gain favor. For each violent act, favor somes in. Even hunting and herding animals gains a small amount of favor. The Norse hero unit is the Hersir, who also creates a small amount of favor just for existing. Each Hersir is given a name when created, by using a random name generator. There exists a great sense of humor in many of the Hersir names. Norse players can also create dwarves at their town centers. Dwarves are excellent at mining gold, but not so good at food or wood. Norse are somewhat handicapped in that they do not possess ranged units capable of pierce damage. Their is only one exception to this, the ballista, built in the mythic age at the Hill Fort.