The screenshot can be of anything you want to show off: a beautiful moment, a hard-earned achievement, a creative triumph, a funny visual glitch, etc. Take a screenshot of your game and then post it here, telling us what game it's from and, more importantly, why you chose that particular moment to highlight. What does it mean to you?
Also, if you're gaming on something that doesn't allow for easy screenshotting (e.g. older hardware or handhelds), feel free to snap a picture of the screen itself with a camera.