Generally, a press kit is requested in order to enable someone else to produce materials about your product or company. The most common use is by the press for a magazine or newspaper article, although some people simply request one as a matter of protocol in order to learn more about the company.
The more you include in a kit, the more likely that a writer will simply regurgitate what you give them, since it makes their job a lot easier. This is one reason to give them a lot of pictures and copy.
A press kit usually has things like:
* A company overview, including management and perhaps some history
* Pictures of management (high enough resolution for magazine use)
* Datasheets of products
* Pictures of products (again, high rez)
* Whatever marketing drivel you want to give in the form of whitepapers, etc.
For a small business, you may not have all of these things, but a good kit can make you look a bit larger than you actually are, and is a good way to get your own writing into someone else's article by making their job easy.
Also remember that some folks request kits just as a matter of protocol, when researching a company. This can include casual passers-by and competitors. Don't put anything in the press kit that isn't information you want completely public.