What's The Difference Between A Caucus And A Primary?

In the U.S. electoral system, states decide on how they nominate candidates. Why do some states have caucuses while others have primaries?

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The Caucus System in the U.S. Presidential Nominating Process (cfr.org)
"Political party caucuses, once the dominant system for choosing candidates in the presidential nominating process, have been less influential than primary elections for decades."

What are the Iowa caucuses? (vox.com)
"The Iowa caucuses are the first time actual voters all across any U.S. state get up and go say who they want to be president."

The problems with caucuses and conventions (washingtonpost.com)
"The Republican Party recently changed its presidential nomination procedures, mainly by adjusting the calendar of caucus and primaries."