iDisk is a WebDAV share at mac.com
You get 100 megabytes of online storage when you sign up for .Mac. It's called iDisk since everything these days has to have an 'i' prefix. (On the other hand, remember 'e-business', 'e-commerce', etc.? But that's neither here nor there.)
You can check to see if you're exceeding your iDisk capacity by checking the iDisk tab of your Internet Preferences. There's a nifty 'fuel guage' widget that gives you immediate feedback.
Other than that, I haven't had much problems with Backup. I just use the default settings, plus I add my Mozilla bookmarks file. For anything more serious, I have an external PCMCIA drive (8 GB).
Tom Sinclair
"Everybody is someone else's weirdo."
- E. Dijkstra