What is an ".abbu" file and what does it do?
A file showed up on my esktop named "Address Book -......abbu". What is it and can I delete it?
It is an Address Book archive file. You can make one by selecting File>Export>Archive while in the Address Book app. It has a date next to the name to indicate when it was made.
I always keep a recent one of these archive files in a backup folder, together with similar archives for iCal, Google Earth Places, Safari Bookmarks etc. They can be a godsend when things go wrong.