The ability to quickly save the content of your screen is a useful feature to have, especially if you are troubleshooting aspects of your system. If an error window pops up on screen, instead of describing it to a technician, it helps to snap a screenshot and send that to better show exactly what is going on.
There are several options for creating screenshots in OS X, including Apple's "Grab" utility and the common keyboard shortcuts for doing so, but in addition to these, you can also use Preview. Generally this tool is used for viewing PDFs and image files, but it can also be used to scan documents, import files from cameras, and grab various types of screenshots.
To use Preview's screenshots function, open the program and go to the File > Take Screen Shot menu, followed by selecting one of the three options for taking a screenshot:
This will display a crosshair cursor, which you can use to grab a section of your screen to include in the screenshot. This is great if you want to use only a portion of the screen that includes multiple windows, without taking a full screenshot and then cropping at a later point.
This option will make the mouse cursor highlight various windows onscreen, and when you click them, it will take a snapshot of the selected window. This is convenient for creating a full image of a single window, including its surrounding shadow, which can be nice for making presentations of screenshots.
From Entire Screen
This will take a full picture of the whole screen, which you can then select from and crop according to your needs. This is the classic screenshot option that most people may be accustomed to.
When these functions in Preview are invoked, the resulting image will be placed in a new window from where you can annotate, crop, or otherwise edit with Preview's various utilities, and then save in the format of your choosing (PNG is the default).
Preview's screenshot capabilities may be convenient, but if you want a full set of screenshot options then your best bet is to either use Apple's keyboard shortcuts or the Grab utility (available in the /Applications/Utilities/ folder).
Shift-Command-3: The basic screenshot will take an image of the full screen.
Shift-Command-4: The "selection" mode will take a shot of the area outlined by dragging the cursor.
The Shift-Command-4 keystroke has some additional options once invoked. If you press the space bar when the selection cursor appears, you can click individual windows (even if they are not in focus) to take a screenshot of them. In addition, if you start a selection of the screen with the cursor, you can adjust its location by holding the space key when the selection box is on the screen. With the space key held, moving the mouse will move the current size of the selection box around on the screen. Releasing the space key will then allow the selection box to continue to be resized.
Similar to holding the space bar with the selection box present on screen, if you hold the Shift key then you can adjust the size of the selection box either vertically or horizontally (determined by the initial movement of the mouse after the Shift key is held). As with the other screenshot options, releasing the mouse will result in the screenshot being taken, but if you press the Escape key then you will cancel the screenshot process.
Another convenient feature for any of the keyboard screenshot combinations is to include the Control key in the key sequence, which will put the resulting screenshot in the clipboard instead of placing it on the desktop.
If you memorize them, these keyboard shortcuts allow you to take shots of selections, windows, or the full screen and save them either as a file to the desktop or to the clipboard for pasting into any program you wish, but if you forget them then most of the common screenshot options can be done by launching Preview.
One last option to keep in mind is if you would like to keep the OS X cursor in the screenshot (useful for illustrative purposes), then using the Grab utility is your best bet. This is especially true if the cursor you want to include is a nonstandard one like the hand pointer, a crosshair, or alternate forms of the standard pointer; the Grab utility allows you to specify the type of pointer to include from a selection of common ones.