What are the differences between air brakes, spoilers and lift dumpers?
Air brakes increase drag with little change in lift. The following image shows the air brake being used in BAe 146



"Eurowings bae146-300 d-aewb arp" by Adrian Pingstone - Own work. Licensed under Public Domain via Commons.

Spoilers both increase drag and reduce lift- basically, they change the lift-to-drag ratio. Sometimes, they are also used as control surfaces in lieu of ailerons. This image shows a Boeing 777 spoilers being used in flight.



Screengrab from youtube.com

The lift dumpers are basically ground spoilers that are used to 'dump' lift, especially immediately after landing, where you want rapid reduction in lift. The following image shows lift dumpers being used in BAe 146.