The Holga 120S is the basic version of the medium format 'toy' camera that has recently become ultra popular. The camera is poorly made, everything in it is plastic, including the lens and this leads to blown out, wild looking photos. The worst optics ever produce some of the most striking images- this is lomography at its best.
You can get Holgas moddified to do different things by giving them different features. For instance some modded Holgas can shoot 6*6 and some only 6*4.5, others have been modded to include cable releases, tripod mounts, and even to shoot 35mm. Holgamods.com explains the common mods and what they do. toycamera.com  is a very active toy camera (Holga, Diana, what have you) crew with forums, tips, galleries, etc.
Loading film into the camera is notoriously difficult and its often hard to tell whether its loaded correctly until the whole roll has been shot and developed. The back is also prone to falling off half way through a shoot, exposing the film, so its a good idea to tie the back on using velcro or elastic bands. The shutter button is placed beside the lens as opposed to the standard place which can be confusing for first time shooters.
Recently Holgas have become available in a kit form which acknowledges and even celebrates their low-tech nature. The kit includes a manual that details the camera's idiosyncrasies, as well as a roll of black tape for taping up light leaks -- though the manual notes that many Holga devotees regard light leaks as part of the camera's signature style.