What all does ADB backup and how do I restore part of it?
For what ADB backup does, you might wish to see our backup tag-wiki. Basically, depending on the command line switches used, it backs up apps (.apk) and their data. For a full list of options see this.

As for the second part of your question, you might wish to see Is there a way to look inside and modify an adb backup created file? Yes, it's possible extracting parts of it, and yes, Titanium Backup is one of the apps capable of this. Other ways are described in the linked question.

Also worth a look:


How do you extract an App's data from a full backup made through “adb backup”?
Partial adb restore
Backup and restore Android Settings and Application through ADB