Capitalism is an economic system in which industry and trade are controlled by private owners in a bid to make as much profit as possible. Key to the idea of capitalism is the idea of investment, spending money to finance an operation like a sea voyage or a factory to gain more money later on in a cut of the future profits. This sort of approach to gaining wealth was used by some merchants and nobles to some degree since the invention of money, but it really came into its own Western Europe between 1500 to 1800. Capitalism can be considered the polar opposite of Communism, whose tenets center around abolishing currency and ownership rights.
The concept of Capitalism has had an impact on /tg/ like many political systems have, as they are perfect to create races/settings/empires from. Many human and other civilised nations in other settings use capitalist trade and economic ideas in their settings, and often national relations between races depend on existing trade agreements (busty elf maidens need to get paid somehow!).
The Card Game
Capitalism is also the name of a card game (also known as Rich Man, Poor Man). Players are dealt cards, and try to discard their hands as quickly as possible. Each trick is set by whoever leads -- for instance, a player can lead with two threes (the card value order runs from three up to two, as opposed to the usual two up to ace), and the next player must play a pair of fours (or fives, or whatever) or pass, and so on. Whoever plays the highest cards in a trick wins it and gets to lead the next, and whoever is out of cards the first becomes the "rich man" (or "president" or "first" or some other title), and so on, until the last player with cards becomes the "poor man". Some variants of the game have twos usable as "bombs" to win a trick (even if the trick is something other than a single card), but players who use a two as their last card immediately become poor man.
Cards are dealt again for the next round, but before play begins, the poor players must trade their highest-value cards to the rich ones (who may give whatever cards they please). For example, in an eight-player game (usually played with two decks of cards), #8 trades three cards with #1, #7 trades 2 with #2, and #6 trades 1 with #3. Hence the name: the rich get richer, and the poor get poorer. The only consolation to the poor is that the poor man leads the first trick.