There are two type of reverse thrusters used on aircraft engines - depending on the type of engine. These are the clamshell type (as with DC9,MD80) and cascade reversers (normally used on high bypass engines as found on 737-300 and later, 747, 757, 767 etc).

Aircarft with the clamshell reversers are able to create enough power in the reverse direction to move the aircraft backwards as has been noted with AA (and other's) use of this technique for "push back".

With the clamshell reversers, the engine exhaust stream (and any bypass stream on a turbofan engine) are deflected back towards the front of the aircraft.

With a cascade reverser engine, only the bypass air is deflected and the exhaust stream continues through the core of the engine. The main use of reverse thrust on landing is not to slow the aircraft directly. Engaging the cascade thrust reversers negates the forward thrust of the engine at flight idle power, and (more importantly) to spoil the airflow over the wing and greatly reduce lift allowing the wheel breaks to operate more efficiently as the aircraft is no longer "flying" down the runway.

So you will generally find that modern high-bypass turbofan engines designed to be slung under the wings will have cascade reversers (the old P&W turbojets on 737-200 are an example of an exception). While turbojet (not many still used on commercial operation - eg DC9) and low bypass turbofan (MD80, 727 etc), especially with high rear moungted engines will use the clamshell reversers.

Cascade reversers can generate sufficient reverse thrust to move the aircraft backways, but as they are generally installed on low slung engines and due to the large air intake area, the risk of damage from FOD (Foreign Object Digestion) is too great and so it is generally not done.

One of the design features of the C17 is the ability to use reverse thrust to manouver on the ground.

Of course, the risk with using reverse thrust for pushback in a rear-mount config like the MD80 and B727 is that applying the wheel breaks too hard causes the aircraft to sit up on its tail! Not something you want to do with an aircraft full of passengers.