With the stsadm backup operation, you can do full or differential backups, not incremental. You select between full and differential using the -backupmethod
parameter.
If you use the -url
parameter to the backup operation, the backups are done at a site collection level. If you use the -item
parameter, you can backup at the content database or web application level.
You'll probably need to do some playing around with the -showtree
parameter to understand the syntax required for the -item
parameter. -showtree
will give you a tree of everything in your server farm, and mark what is going to be backed up if you were to remove the -showtree
parameter.
See this TechNet article, especially the "Correction" community content by Mark Vogt at the bottom of the page for details about how to get this setup.