Primaries and caucuses differ primarily in their voting processes for political party nominations. Primaries are statewide, secret ballot elections where voters choose candidates directly. In contrast, caucuses involve local gatherings where party members discuss and publicly show support for candidates, often leading to a less formal atmosphere. Primaries can be open or closed, affecting who can participate, while caucuses typically restrict participation to registered party members. These differences affect voter turnout and engagement, shaping the overall dynamics of candidate selection and party unity leading up to general elections.