Most people have heard of the Iowa caucus, but very few actually understand how it works.
Most states use primaries to select delegates to the national convention.
In a caucus, people gather at public spaces or in private homes or halls and express their preference in person. The actual procedure differs in the Republican and Democratic parties. The Republicans have people at the caucus put the name of their preferred candidate in a hat, and then the tabulated results are sent to the state party headquarters. This is mostly because the state leaders in Iowa brokered a deal with the state leaders of New Hampshire to make sure each state held the first caucus and primary. If Iowa switched to a primary, then they would either have to move their primary date to after New Hampshire, or broker a new deal with New Hampshire.