Precinct caucuses select uncommitted delegates to county convention which in turn selects delegates to the congressional district convention which in turn selects delegates to the state convention.
Most national delegates aren’t committed to a candidate until the state level convention.
The system favors party regulars or highly organized groups. In 2012, the ‘winners’ were reported to be Santorum and Romney, but the national convention delegates were mostly committed to Ron Paul. That happened because Romney didn’t bother with precinct level organizing.
In 2016, Gov. Branstad political machine is going to be ready.
He was on the county level. I saw it personally!