A press kit, also known as a media kit, is an expansion on your press release. It is usually a high quality folder containing your press materials: press releases, brochures, collateral materials, company biography, news clippings, photographs (if applicable), contact information and any other relevant materials. These are frequently used to attract new clients. They are also made available to the media upon request when someone is seeking more information about your company.
EPKs (or Electronic Press Kits) might also contain videotapes of relevant information (i.e., commercials, news spots, etc.).