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it's a way to tell how well your computer preforms under intensive tasks. it's based on a number (3dmarks via a program called 3dmark06) or it could be as simple as the frame rates received in a popular game. e.g. "80 fps on high in Crysis" can be used as a very general benchmark.

you can view benchmarks for entire systems or through specific parts like cpu and gpu benchmarks.
 
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