Understanding Politics: What's the Difference Between a "Caucus" and a "Primary"?

The Iowa caucuses are tonight, officially launching the 2016 White House race. http://cnn.it/1KR4qhW
  1. Caucus
    It's a neighborhood event that requires several hours of active communal participation and debate, and takes place in the evening in a home or public space, depending on the size of the caucus location.
  2. Primary
    It's what most people traditionally think of when they imagine voting - people show up at a neighborhood polling place to vote for their candidate by ballot.