If you wish to get a screenshot of the whole screen (desktop), use the shortcut Cmd+Shift+ 3 together. If your speakers are turned on, you will hear a camera sound affect and the capture will automatically be saved as a .png file on the desktop. To view the screenshot, just double-click on the file to open it.
To get a screenshot of a part of your desktop, use the shortcut Cmd+Shift+4. A cursor with cross hair will show up that you can drag with your mouse to pick the portion of the screen you want to capture. A screenshot will be captured as soon as you will release the mouse button.
Note: In Mac OS versions 10.3 and earlier, the file will be saved as .pdf
You can also capture a particular application window. To do this, press Cmd + Shift + 4, and then hit the space bar. You will see that the cursor now appears as a camera icon and you can hover it over the screen. As you hover the camera over different items on the desktop, some will seem highlighted. Just press the mouse button to take a screenshot.
Please note that if you wish the screenshots copied to the clipboard and not saved, include the Control key to the shortcuts above, like in this shortcut: Cmd+Control+Shift+3.
There is a very good utility in Mac OS X that also lets you capture screenshots, it is called Grab. You will find Grab in the Applications --> Utilities folder. To take a screenshot with Grab, open the application first and from the Capture menu, choose one of the following capture modes.
Window mode
The Window mode lets you select the window you wish to capture. When you select the Window mode, a window will open requiring you to choose the window you wish to take capture of. Click on the Choose Window button and the screenshot will be taken of that window.
Selection mode
This mode lets you capture a specific portion of the desktop by dragging mouse pointer around it. Grab displays a tooltip showing the size of the area you have selected and as soon as you release the mouse button, the screenshot will be captured and opened in a window.
Screen mode
If you select the Screen mode, a window will open waiting on you to click anywhere on the screen to take screenshot. In this mode, you can capture anything on the screen that is at the front.
Timed Screen
This mode enables you to set up your desktop for capture and when everything is all set, click on the Start Timer button and the timer will start. You will then have 10 secs. before the screenshot is taken. In this wait period, you can adjust the screen items, open any menus, etc to include in the screenshot.