The caucusing process

My wife has asked on several occasions over the past couple months what's the difference between a primary and a caucus. Typically we all know how a primary works - it's just like any other election - while a caucus is different. I was sent this link to a fun, clear description of how the Iowa caucus works - it's from hillaryclinton.com. If you want a less biased version of basically the same information here's Wikipedia's explanation.