Here's the camera in question, inserted into the limited edition Pokemon Gameboy Color. The camera costs about 25 pounds in the UK, or 50 dollars US (at full price - they should be available much cheaper).
The camera is compatible with all Gameboy models, even the ugly old heavy brick-like ones that ate batteries like a redneck at a pie festival.
You can also get a Gameboy printer for a bit more (50 pounds or 70-ish dollars). These are a lot of fun, but not the best way to store pictures (they're thermal paper-based, and not desperately well made). The best thing about them is the wonderful sepia prints you can get on white paper. Great if you have ugly people to take pictures of - you can claim they're friends of your great-grandparents.
A Gameboy camera is capable of holding 30 pictures at once, which can be stills, or frames of an animation. It also comes with a simple built-in music sequencer and a couple of fairly dull games.
These places can tell you more technical detail about the Gameboy camera