As of the Oops, You Did It Again expansion, there are 64 factions, which can combine to form 2,016 unique pairs of factions.[1]
Each player shuffles two factions together to form their deck of 40 cards. Every faction typically has 20 cards with some combination of minion cards and action cards. Factions have a typical structure, although there are many exceptions. The typical structure of a faction's cards is the following:
Each faction has a special gameplay mechanic that is common among the faction's minions and actions. For example, Zombies come back from the discard pile, Ninjas specialize in destroying other minions, and Ghosts gain bonuses from having only a few cards in their hand.
Each faction has two associated bases, released in the same set. You might say that each faction has two bases, although the relation is mainly thematic: If a faction is not chosen, its associated bases can still appear, and some bases don't really give an advantage to their associated faction, e.g. Jungle Oasis (the Dinosaurs can easily score it, but only for 2 VPs), Temple of Goju (by themselves, the Ninjas won't be able to get back their highest-power minion), Cave of Shinies (the Tricksters can destroy other players' minions, but they definitely don't want to do it there, and their own minions are usually protected against destruction), The Dread Gazebo (the Ghosts don't get any advantage from it and they even have a play-on-base action),...
Play extra minions, usually straight from your deck, play cards with delayed effects, usually happening at the start of your next turn, but that will give you more benefits as turns go by.
Play play-on-base actions that benefit you, giving you more power and movement, and salvage discarded actions from your discard pile, especially play-on-base actions.
Give Madness to other players, usually forcing them to choose between drawing one or giving your an advantage, force them to reshuffle those Madness cards into their deck, but on the other hand you have strong minions that are difficult to play.
Copy other players' power and abilities, boost your minions' power, and destroy and replace your minions with other minions, especially from your deck.
Sort your deck, take a peek and manipulate the top of each player's deck, force your opponents to discard cards, and play specials while bases are scoring.
Weaken other players' minions, temporarily take control of weak minions, and destroy your own minions for effects, whether minions you played or minions you took control of.
Play with a reduced number of minions in your deck, but with a cast of six power-5 minions with useful abilities, and support your deck with movement, card draw and minion retrieval.
Draw treasures, gain benefits for having treasures in hand, usually discarding them for the benefits, and steal things from other players, such as cards or VPs.
Play and destroy monsters on bases to manipulate their breakpoints and gain treasures, and gain power boosts from the presence or the destruction of monsters.
Play play-on-minion actions, use talents to switch your minions between a powerful form and a weaker form with a useful ability, and move your minions around.
Return your minions to hand for benefits, and manipulate how your minions are affected, like moving them instead of returning them or having your power-2 minions be affected instead of others.
Play low-power minions as extras, gain benefits from the presence of other players' minions on their bases, and play actions that neutralize strong opposing minions.
Play with fewer minions than normal, but gain power through your play-on-base actions, destroy other players' cards, manipulate bases and summon a giant freaking titan.
Transfer play-on-base actions around in order to move your minions, and mess with other players' play-on-base actions, either by transferring them elsewhere or taking control of them.
Play with a reduced number of minions in your deck, but with a team of seven power-4 minions with powerful and oppressive abilities, including hard removal, power boost and retaliation.
Draw and retrieve cards from your deck and discard pile to get your best cards out as soon as possible and keep them around for as long as possible. (Previously found in the All Stars Event Kit)
Search for and play actions on other players' minions, especially Set-Up actions, as a requirement to activate or play cards with much greater effects.
Edit
