Quite often, when people ask me for advice on an issue they have with their Mac, I will ask them to send me a screenshot with the error message that is displayed in order to help me troubleshoot that bit quicker. More often than not, I will actually be asked how to do this as it doesn't appear to be common knowledge. There are many third party tools you can use for this however OSX has two tools built-in which I will outline now.
If you go to Spotlight in the top right hand corner and type the word Grab, click the Application in the resulting set of search results and you should see the word Grab appear in the top left hand corner of your screen, indicating that the application is running and waiting for an input.
If you click on the Capture menu, you will see four options - Selection, Window, Screen and Timed Screen. The two I find myself using the most are Selection and Window.
If you choose Selection, the cursor will change into a crosshair, ready for you to click and drag over a section of the screen. A dialog box will then open prompting for you to choose where you would like to save the resulting file.
If you choose Window, this will let you take a screenshot of whichever Window you subsequently click on which is slightly less real--estate than taking a screenshot of the whole screen.
Screen, as you would expect, takes an image of the whole screen. The mouse cursor will be captured at the position where you click.
Timed screen will take an image of the whole screen ten seconds after you click the OK button of the dialog box that appears. This is useful if you need to open some menus and submenus for capturing.
A useful keyboard shortcut for a quick screen grab is CMD+SHIFT+4. Once you press these keys together, the cursor will turn into a crosshair, the same as the Selection option mentioned earlier. You can either drag a portion of the screen for grabbing (Selection) or press the Space bar to capture the active window (Window)
I hope these hints help some of you out there!