In my Picasa Web library, a new directory "Auto backup" has appeared. It contains several albums (one per shooting time).
What is the difference between this and "Instant upload" (that still exists)?
With the latest change to Google+, it looks like "Instant Upload" has been renamed "Auto Backup".
I'm not sure why they didn't just rename the directory. Maybe in case people had made changes. In my Picasaweb, at least, both "Auto Backup" and "Instant Upload" have the same date of last change on them, but "Instant Upload" is empty and is shared only with me.
In Google+ Photos (which is replacing Picasaweb) "Auto Backup" is put in its own category and not with the rest of the albums. "Instant Upload" is nowhere to be seen.
So, to answer your question: "Auto Backup" is now what "Instant Upload" used to be. The "Instant Upload" album seems to be kept around for legacy reasons.