While there are a number of programs with which you can take screenshots of your computer screen, we have found that the best program is Jing. It's a free program for both Windows and Mac that also includes a free online storage account to which you can automatically annotate and then upload your screenshots (or <5 minute videos). Once it uploads (or saves to your computer), you'll then get a link to send to whomever needs it.
Alternately if you're able to attach files to whomever you wish to have the screenshot, you can use your OS's default screen capture feature.
There are two types of screen captures you can take: the entire screen, or just the active window. For example, if you have three programs open at the same time, you might want a screen capture of only the active window.
1) You can take a picture of what is on your screen by pressing PRINT SCREEN on your keyboard (PRTSC or PRTSCN on some keyboards).
This captures the active screen and copies it to your clipboard.
2) Paste the screen capture into a word document, e‑mail message, or other a program that allows you to save the images as a JPG or PNG file.
You can then attach that file to an email, or drop it into a chat window, to share with another person.
Check out Microsoft's detailed instructions for Windows 8.1 and higher
1) Press Command (⌘)-Shift-4
2) Click and drag the crosshair pointer over the desired capture area.
3) When you have the entire area you wish to capture within the selection, release your mouse.
The image of the screen (screenshots) will be saved as a file on the desktop (called "Screenshot XX-XX-XX" where the Xs are the day's date).
If you prefer to put a screenshot in the Clipboard, just hold down the Control key while you press the other capture key combo. You can then paste the picture into a document. Check out Apple's detailed instructions for OS X 10.6 and higher