A caucus is a meeting of members from the same political party, typically to select delegates to a convention or vote on preferences for candidates running for office.
Not all states have caucuses. The only ones that do are Alaska, Colorado, Hawaii, Iowa, Kansas, Maine, Minnesota, Nevada, North Dakota and Wyoming.
The Iowa Caucus is one of the more famous caucuses. Typically, Iowa is the first caucus to be held, which is also why you see candidates frequent there often. If the candidate is able to win that caucus, then they can start off very strong.
A primary is a statewide voting process in which voters cast ballots for their preferred candidates. This is held before the general election to determine who will be moving on to the general election. Primaries are held in all the states that don’t have caucuses. Some primaries only allow registered party members to vote (closed primary) and some allow all residents (open primary).
The New Hampshire primary is the first primary held every presidential election, which also makes it very crucial one to win. This primary, because of timing and media attention, is said to make or break a candidate. Before 1992, the person elected president has always carried the New Hampshire primary. However, the last three Presidents (Bill Clinton, George W. Bush and Barack Obama) finished second.