Whats i primary help??

i have to write a paper on virginia presidential primaries and i have to choose one person from democrats and one from republicans i chose hilary clinton and rudy giulaiani, my paper with the intructions says this "introductory paragraph: state the fact that there is a primary this year and what the purpose of the primary is; the candidate name; general backround info of the candidate." but im confused on what a primary is and im not sure how to write a paragraph on each candidate on "primary" can some one please explain to me what it is????