There are few options in capturing a screenshot in MAC OS X. These are as follows:

1. To take a picture of the whole screen, press Command-Shift-3.
2. To take a picture of part of the screen, press Command-Shift-4 and drag to select the area you want.
3. To copy the screen capture to the Clipboard instead of saving it as a file, press Control-Command-Shift-3 to copy the entire screen to the Clipboard.
4. Control-Command-Shift-4 to copy the selected range to the Clipboard.

Note: Using options 1 & 2 will make screenshots saved as TIFF Format, also considered as a RAW file, which usually is bigger than JPEG files because it is uncompressed. MAC OS X 10.2 and later versions will save file as PDF.