"Best format" will depend a bit on your audience. For Windows users I'd say your best bet is Windows Media format (a .wmv file) or for Mac users you might want to do QuickTime. If you expect both, then maybe produce both so people have a choice. Just keep download speed in mind, although your clips will be small.

Streaming video means, among other things, that the video can begin playing before the entire file downloads. Your clips are small so you needn't worry about it, and besides, there is technology involved on the server side to make streaming work.