Screenshot capture in MacOS

1. To capture the entire desktop, press Command-Shift-3. The screen shot will be automatically saved as a PNG file on your desktop.
2. To copy the entire desktop, press Command-Control-Shift-3. The screen shot will be placed on your clipboard for you to paste into another program.
3. To capture a portion of the desktop, press Command-Shift-4. A cross-hair cursor will appear and you can click and drag to select the area you wish to capture. When you release the mouse button, the screen shot will be automatically saved as a PNG file on your desktop. (The file is saved as PDF in Mac OS 10.3 and earlier.)
4. To capture a specific application window, press Command-Shift-4, then press the Spacebar. The cursor will change to a camera, and you can move it around the screen. As you move the cursor over an application window, the window will be highlighted. The entire window does not need to be visible for you to capture it. When you have the cursor over a window you want to capture, just click the mouse button and the screen shot will be saved as a PNG file on your desktop. (The file is saved as PDF in Mac OS 10.3 and earlier.)
5. Add Control to the two shortcuts above to place the screen shot on the clipboard instead of saving it to the desktop.
6. Another method for capturing screen shots in Mac OS X is by using the bundled Apple utility, Grab, located in the Applications > Utilities folder. Grab is useful if you need to include a cursor or a menu in your screen shot, or if you want to save your screen shot to TIFF format. To include a cursor, first go to Grab Preferences and select the cursor icon you wish to have in your screen shot. To capture the screen with Grab, run Grab, then choose of the capture modes from the "Capture" menu: Selection, Window, Screen, Timed Screen.
7. When you choose the Selection mode in Grab, you can capture a specific region of the screen by dragging around it. Grab will display a tooltip showing the size of the region you have selected and the screen shot will open in a window when you release the mouse button. The cursor will not be included.
8. When you choose the Window mode in Grab, an instruction window will appear asking you to select the window you wish to capture, then click the "Choose Window" button. When you click the button, the instructions will disappear and the window you click ill be captured, including the mouse cursor at the position where you click (if a cursor was selected in Preferences).
9. When you choose the Screen mode in Grab, an instruction window will appear asking you to click the screen when you are ready to capture. The mouse cursor will be included in your screen shot at the position where you click (if a cursor was selected in Preferences).
10. When you choose the Timed Screen mode in Grab, an instruction window will appear, allowing you to prepare your screen for capture. When you are ready, press the "Start Timer" button and you will have ten seconds before the screen is captured. This allows you to open menus and sub-menus, if necessary. After ten seconds the entire screen will be captured. The mouse cursor will be included in your screen shot if a cursor was selected in Preferences.

[ Source : http://graphicssoft.about.com/od/screencapturemac/ht/macscreenshot.htm ]