Political parties are organized to support and advance a particular platform, or group of principles. In the U.S., there are several parties including the Libertarian party, Green party and Constitution party; However, the dominant parties in U.S. politics are the Republicans and Democrats.
Each party typically has national, state and local organizations which help choose, fund and support party candidates in various elections.
Individually, members may have varying beliefs, but tend to identify closer to the ideas of one party as a whole over another party. A November 2011 Rasmussen poll showed that 35% of Americans adults identified themselves as Democrats, and 34% Republicans. The remaining 31% would be either independent (no political party affiliation) or members of other parties.
The Republican party supports a conservative platform including smaller government and lower taxes, pro-business regulations, pro-life and anti-gay rights, and individual responsibility.
The Democratic party has a more liberal or progressive platform including strengthening federal programs to help the poor and disadvantaged (including universal healthcare), pro-labor policies, strong environmental regulations, and pro-choice and gay rights.