Definition: what a press kit does
At C.S. Lewis & Co. Publicists, the press kit (the firm also calls it a media kit) is the reference that television and radio producers, prominent journalists, print and online editors, and bloggers use for background material, reviews, story ideas, interview topics, and bylined articles.
It is not an ad. It is a working document that answers the questions a busy producer has before booking a guest: who is this, why now, what will they talk about, and who do I call. A good kit answers all four in minutes, and makes the author easy to picture in a segment or a feature.