Generally speaking, a caucus is an event where party members gather to conduct party business. In a presidential year, that can include electing delegates to the next step of the multilevel national delegate election process — that is, the selection of the actual people who will go to the county convention, which then picks delegates for the state convention, where the national delegates are then chosen. How a caucus functions depends on the state and the party's rules. In some states, like Iowa, a caucus includes a presidential vote by participants that allocates national delegate slots to candidates that are then filled by the selection process. In others, the state party may use a caucus-convention system to select the individuals who'll serve as delegates, but actually allocate national delegate slots using the state's presidential primary.
Today in Iowa, Republicans are holding precinct caucuses to begin their national delegate selection process (Iowa Democrats will also hold them, but no longer use them to allocate national delegates). Run by the party, the GOP meetings take place in the evening at government centers, schools, churches, businesses and even homes. Prior to the preference vote, the caucus chair at each caucus will invite one attendee each to speak on behalf of each of the candidates. Then, each attendee casts a presidential preference vote, which gets reported to the wider world. The results of these votes statewide will determine how the state's 40 national delegates are allocated. (Before 2016, this much-watched vote was actually non-binding and played no direct role in the Republican allocation process.) Following the preference vote, attendees can stay to participate in party business, including electing precinct committee members and selecting county convention delegates.
While most states hold primaries, Iowa Republicans are not alone in using a caucus vote to allocate delegates: Seven states lack a presidential primary law, and most territories use a caucus-convention or other party-run event, too. Even in primary states, a party can forgo an available primary option to allocate delegates with a party-organized event, such as a caucus or party-run primary. Nevada is a prominent example this cycle: There, the state GOP opted to use caucuses on Feb. 8 to allocate delegates instead of the state's primary on Feb. 6, although a Republican primary will still occur.