They are pretty much the same, in function. Essentially, they are devices that hold more than one tape in slots. A robotic device of some sort then loads one tape at a time into the drive. When that tape is full, it is replaced in its slot and the robot loads the next tape. Basically, they just remove the need for human intervention in order to change tapes if one fills up. In my experience, devices classified as "autoloaders" tend to hold a smaller number of tapes.
Say, for example, you have a library or autoloader with ten slots. Each of those slots might have one 80 GB tape in it. For the sake of simplicity, we will say this device has one tape drive. Now, if you kick off a backup job that will use 120 GB of tape space, the device will load the first tape and write 80 GB to it, eject it, and load the next tape to write the remaining 40 GB. This requires no action on your part to complete. Backup Exec has built-in features for organizing and managing your tapes, slots, and drives.
