1) non-electronic -- this would be the kind for most formats, and tend to be cheaper.
2) electronic -- passes signaling through, so you can auto-focus, for example.
Examples of the former would be the Canon FD adapters; the latter would be the 4/3 adapters from Olympus (MMF-1 and MMF-2) and Panasonic.
The number of adapters available for micro-4/3 is so dizzying I'm not sure a complete compendium could be made. Suffice to say they'll take almost anything that was in production for any significant length of time.