How can I take multiple screenshots from a PC game during playing?
The depends 100% on the game in question.
However, if your game doesn't natively support it, FRAPS is probably your best solution.
Since the other answer covers only the Windows PC operating system, here are a few more methods for other PC operating systems.
Mac OS X
Grab is one way to get screenshots and window captures:
This varies greatly by the flavor you install. Often there may be a utility simply called "Screenshot".