Mac OS X has several built-in shortcuts to take screenshots. Since Mac OS X 10.4 (Tiger) screenshots are saved in png format.

• Take screenshot of  the whole screen
  Command + Shift + 3
• Take screenshot of selected area
  Command + Shift + 4 > select area
• Take screenshot of a selected Window
  Command + Shift + 4 > press Space > select Window

Screenshot files will be saved on the Desktop.

There also are some terminal commands that can change various screenshot parameters. For changes to take effect you may need to run: killall SystemUIServer (or log-off and log back on)

• Change screenshot format (default: png):
  defaults write com.apple.screencapture type <format>
  where <format> is one of the following formats: jpg, png, bmp, tiff, pdf, pict
• Disable of enable shadows (default: false):
  defaults write com.apple.screencapture disable-shadow <true | false>
• Change where files are saved (default: ~/Desktop)
  defaults write com.apple.screencapture location <path>
• Change screenshot file name prefix (default: “Screen Shot”):
  defaults write com.apple.screencapture name “Prefix Name”
  

Mac OS X 10.8