The bayonet mechanism and the electrical contacts are visible in this shot of the Canon EOS 1000D with its lens removed.
Each maker produces its own proprietary lens mount which is incompatible with the rest. You can't fit Canon lenses to Nikons or Olympus lenses to Pentaxes. There may even be some incompatibilities within the maker's own range. Canon's EF full-frame lenses can be used on its full-frame cameras and it's smaller APS-C models. However, the EF-S lenses designed for the APS-C models can't be used on the full frame EOS SLRs.
Manufacturers are constantly developing their camera and lens ranges, and it's always wise to check compatibility with your existing equipment before you buy, or that your existing lenses can take full advantage of the features in the latest bodies, say.
Camera makers only make lenses for their own cameras, but independent lens makers like Tamron and Sigma may offer their lenses in several different mounts. When you buy an independent lens, you must specify the lens mount you want.
