A screenshot is an image of... well, your computer screen. It's a snapshot of what you are looking at on your monitor.
A screenshot may also be called a screen grab, screencap, screen capture, screen dump, or print screen. Screenshots are often used (and sometimes cropped) on this blog.
Here is a screenshot I took while writing this entry:
You'll see the blogger interface, my other open tabs, and my browser.
A screenshot taken on a Windows machine will go to the clipboard after it's taken. (It can be pasted and saved.) You can take your screenshot by pressing the PRINT SCREEN button on your keyboard.
A screenshot taken on a Mac will save to the machine as an image file. You can take a screen shot by pressing these three keys at the same time: SHIFT + COMMAND + 3.
You can also take screenshots on your Android, iPhone, iPod, and Maemo 5 (which is a Linux-based system on the Nokia N900). In fact, most screen devices have this function.
Screenshots are pretty basic and very useful.