One option for Windows users is that little PrtScn button floating around somewhere in the upper right part of your keyboard. When you press it, it puts a picture of the whole screen on the Clipboard (if you use two monitors, the picture even includes both screens). If you want just the currently active window (which is far more likely), press Alt-PrtScn. Then you can paste (Ctrl+V) the image in an email, a Word document, or wherever you want it. If PrtScn isn’t convenient enough, there are other options.
My favorite is a little program for this job is called FastStone Capture. It gives you great options such as grabbing only a portion of the screen, a particular window, the option to open the screenshot in a built in editor (or one of your choice), the format the picture is saved in, where to save the image, and the list goes on. All the settings can be accessed by clicking on the clipboard-looking icon at the far right of the program and choosing Settings.
If you want to give it a test run you can grab a copy here. It runs from a file so there is no need to install; just unzip the download and fire up FSCapture.exe. You’ll see a small window that looks like the one above. Hovering over the various icons tells you what each one does (various ways to take screen-shots). Find the one you want and click on it to get started. If you like it and want some more depth they have an official tutorial, look here.