A pinhole camera is a very simple device. You poke a very small pinhole into the outside of some light-safe object, sometimes round and sometimes flat-backed. Inside, you place film, with a cover over the hole. Then, close the case, open the hole to expose the film, and cover the hole again. Then process the film!
This works on a simple principle. Narrow, small holes will function as a lens by inverting light and then projecting it forward. Even old keyholes do this! Because the hole is so small, it functions as a very small aperture, allowing for maximum depth of field so that almost everything will be in focus. As a result, it works best in very bright conditions. Otherwise, you would need extremely long exposures.
This property of small holes was first exploited in the Camera Obscura (which is where the name "camera" comes from.) Camera means "room" and obscura means "dark." So, a camera obscura is a "dark room." Even a small camera that you might hold in your hand is still a "camera obscura." It's just too small for you to fit in it!
OK. So, now that I've explained that, hopefully you're excited about the possibility of making one of these suckers yourself! A pinhole camera, that is. Well, Photojojo has supplied a few super cool designs for free pinhole cameras! OK, the thing does require materials which will somehow cost you something, but the idea and designs won't cost you a cent!
Make Pinhole Pictures! Then, find the right Flickr Group to post your magnificent images.