There are a lot of reasons to want to capture a snapshot of your screen. You may want to preserve something to share it with your students. You may want to send it as an e-mail attachment to a friend or co-worker. Or you may just want to save it as a prompt to your memory so you can return to it later.
No matter the reason, there are several options for you to choose.
The easiest is the built-in screen capture command on the Mac. Pressing COMMAND+SHIFT+3 will take a photo of your desktop. Couldn't be easier.
But perhaps you want to take a photo of just a portion of your desktop...what to do? Press COMMAND+SHIFT+4. Your cursor will turn into cross hairs which you can drag and select an area of the screen. When you drag and release your mouse, the photo will be taken of the selected area.
Want more options? Check in your Utilities folder for Grab. Using that application will allow you to take a photo of the desktop or a selection, just like before. But it also adds the options of taking a snapshot of a window and of taking a timed capture. The timing feature gives you a chance to arrange things on the screen including pulling down menus for the capture.
Finally, I love using Skitch for screen captures. It gives you all the options of Grab plus it allows you to edit the photos right in the software before saving them to your desktop.
Using one of these options, you should be able to grab just the screen capture you need without any problems at all.