Caucuses vary by state and political party, but Iowa, the first state to participate in the presidential nominating process, sets a good general example of how the procedure works.
A caucus is generally a meeting place for members of a political party to discuss and decide on policy and legislative candidates. It is not as simple as sitting in a voting booth and dropping off a ticket.
Registered Iowan voters from 1,774 precincts are meeting across hundreds of caucus locations to elect representatives Tuesday night.
The Huffington Post describes what this may look like:
“When Republican representatives, friends, neighbors, and family members convene in Iowa gymnasiums, church basements, and high schools tonight they will also see something else: representatives for candidates. The caucus process allows candidate representatives to attempt to persuade undecided caucus-goers at the last minute, in many cases even hearing impassioned speeches before the group comes together for a vote. The same typically isn't true for primaries.”
These representatives then go to one of Iowa’s 99 counties to choose more candidates who will represent their county district at the state level.
This cycle generally continues all the way to the national level at their party’s national convention.
The two major national conventions are the Democratic National Convention and the Republican National Convention.