Polly Farrington has just done a terrific slidecast of the differences between screencasting and slidecasting. I know, there's a lot of "s" words in that sentence. Try saying "screencast, slidecast" fast three times. :)
Anyway, take a look at Polly's terrific slidecast. I've been meaning to play around with slidecasting capabilities on Slideshare.net for awhile now and Polly's slidecast is a great introduction.