Decent overview of the difference between primary vs. caucuses

https://www.dictionary.com/e/caucus-vs-primary/

Note:

Today, state party caucuses are open to anyone who is a member of the party and registered to vote. These members meet to decide on who they will recommend for nomination, ultimately at their party’s convention. They also discuss the party platform and organize election volunteers. 

Caucuses are very different from primaries because they require that voters go to a meeting to participate in the nominating process. And this means caucuses can get pretty lively at local precincts. In Iowa, voters literally try to persuade each other in small groups.