There is no difference. To abseil and to rappel are the same thing. Both are rock climbing terms.

Listed below are some rappelling (a.k.a. abseiling) devices that you can use.

Bobbins
The rope follows an “S” path around two bollards. True bobbins have the center of each bollard fixed to the frame. They come in plain and stop versions. False bobbins have bollards whose centers move with respect to the frame.

Carabiner Methods
Rappels using carabiners only or carabiners and brake bars only.

Figure Eights
Devices that look like an “8.” Deaf eights don’t have ears; obviously eights with ears do. There are complex eights with moving parts, and toy eights that are not intended for rappelling.

Fixed Multi-bar Devices
The rope snakes around at least three bollards, all of which are fixed.

Hooks
An ill-defined catch-all category, but generally the rope wraps around a rod.

Horns
Another ill-defined “catch-all” category for devices with horns or prongs.

Poly-bollards or Fixed Multi-bar Device
Descriptive terminology for the rope that snakes around at least three bollards, at least one of which moves to provide a stop function.

Racks
Devices with frames that accept a number of brake bars, at least some of which can move on the frame. J-frame racks have an open side, U-frame racks do not.

Spools
Devices where the rope wraps around a drum. The drum axis can be horizontal or vertical.

Lever Boxes
Devices with an enclosed rope channel and a control lever.

source : http://www.xtremesport4u.com/2008/06/11/what-is-the-difference-between-abseiling-and-rappelling/

http://www.youtube.com/watch?v=AnPgeeOGgQY