A great review of this subject comes from a really interesting article at PBS. Take a look at this excerpt:
In what’s thought of as a traditional primary, political candidates only compete against other contenders within their own parties for nominations, to then advance to the general election. States hold their primaries on a variety of dates, with winners competing with one another on the November ballot.
But in a “jungle primary” or “majority vote primary,” all candidates regardless of party run against each other on the same ballot. If no one candidate tops 50% in that primary, the top two vote-getters advance to a head-to-head runoff, which can end up pitting two Republicans or two Democrats against each other.