Jacob Kaplan-Moss recently wrote a series of really good articles about writing good documentation, I highly recommend reading them if you're about to embark on making technical docs:

http://jacobian.org/writing/great-documentation/

He subdivides documentation as a whole into more or less the following three sections:

- Tutorials

- Topical Guides

- Reference

Screencasts can work for Tutorials, and they can be used for marketing purposes, in which they're closer to topical guides but probably won't provide as much depth as a textual guide could.

Imho. a screencast can be great as a marketing tool, but not so much as a learning tool.