Most operating systems have certain methods to capture a screenshot, or an image of the contents currently on your screen. On some it is more obvious than others, such as Windows, where most keyboards come with a "Print Screen" key that you can use to paste into a graphics program like Paint. On the Mac operating system, it works quite differently. You can take many different types of screenshots, as opposed to just capturing your entire screen, and each screenshot is taken with a certain key combination.
Take a screenshot of your entire visible screen by pressing the "Shift," "Command" and "3" keys simultaneously. Upon releasing the keys, the screenshot will appear on your desktop as a new file.
Take a screenshot of a single window by pressing "Shift," "Command" and "4" simultaneously. When the crosshair selection appears, press the spacebar, at which point a camera icon will appear. Mouse over various windows; they will turn blue, showing you which window will be captured. Click your mouse over the window to take a screenshot of that single window. The screenshot will appear on your desktop.
Take a custom screenshot of any area on your screen by pressing "Shift," "Command" and "4" simultaneously. When the crosshair icon appears, click and hold down the mouse button, then drag over the area you want to capture. A box will appear as you drag, showing you what area will be captured. Release the mouse button to create your screenshot, which will appear on your desktop.