Baitcaster reels belong to the family of "overhead reels" which are simply reels that sit on top of the rod instead of below it (eg, egg beaters/alveys).
Overhead reels are then broken down into 2 categories, ones which you can cast and ones which cant.
The smallest of the castable reels would be your baitcasters, and the bigger ones would be your overhead spinning reels.
The basic design is the same, ie, the line directly off the spool and not at a right angles. Most usually have a level wind to lay the line evenly on the spool and some system to prevent spool over-runs when casting (birdsnests)
The overhead reels that cant cast are usually designed to store much more line to catch bigger fish in scenario's where casting isnt really required such as off boats or deep water ledges.
Hope this helps. =)