measure game framerate?

i often hear of people taking note of the framerate they get in games, but i never knew how it was done. is there an app that measures this, or is it something that is done on a per-game basis by entering something into the console?
i can’t tell much difference in speed when i change resolution in games, so i’m curious to know how much the framerate is actually changing…
Answer #1
fraps is the best app for this
Answer #2
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