In February, Primaries and Caucuses, which are the main voting events, culminate in the choosing of candidates to represent parties at conventions. Iowa, New Hampshire, Nevada, and South Carolina play a pivotal role here because the results from those states usually indicate who the final presidential nominee will be for each party
Primary: in the primary election, party members (U.S. citizens who chose to affiliate with a certain party) vote for the candidate who they want representing their party in the general election.
Caucus: in a caucus, party members chose the best candidate who they want to represent their party in the general election, through discussions and votes.