Thrusts are large planar structures that accommodate crustal shortening. In other words, they are the rupture planes thanks to which rock units pile up to form mountain belts (similarly to a pile of cards).

Here is one of my favourite examples, from the Flinder Ranges, in Australia. As clearly visible from the picture below, there is a sharp change in topography going from the plain to the ranges. This abrupt change is related to the presence of the Paralana thrust fault.

Flinder Ranges east of Arkaroola, seen from the Skrzynski Desert. The Paralana thrust fault runs at the bottom of the slopes 

 
The Paralana thrust fault is a rupture plane along which the Fliders Ranges are carried over the plains. Movement along this thrust results in thickening of the crust and in the creation of topographic relief.                      

Outcrop of the Paralana fault. The crystalline rocks of the Flinders ranges (in red) are thrusted over the alluvial sediments of the plain (in white) 

 
This is the Glarus Thrust, in Switzerland, juxtaposing older rocks, of Permian age (older than 250 million years) over younger sediments that were deposited in the Mesozoic. This spectacular thrust can be seen clearly in landscape pictures, due to its sharpness.                      

    The Glarus thrust in the slopes near Elm, the famous ski resort                       

      The Glarus thrust                      

If you want to know more about thrusts, have a look at www.see.leeds.ac.uk/structure/faults/thrust/index.htm

For some more spectacular images of thrusts visit:

http://rst.gsfc.nasa.gov/Sect2/chileThrust.jpg