There are lot of screen-capture tools, but a lot of them are based on X(GUI). Sometimes, when running an X application may interfere with the application you wanted to grab – perhaps a game or even a Linux installer. If you use the venerable ImageMagick import command though, you can grab from an X session via the console(terminal). Simply go to a virtual terminal (Ctrl+Alt+F1 for example) and enter the following:
chvt 7; sleep 2; import -display :0.0 -window root sshot1.png; chvt 1;
The chvt command changes the virtual terminal, and the sleep command gives it a while to redraw the screen. The import command then captures the whole display and saves it to a file before the final chvt command sticks you back in the virtual terminal again. Make sure you type the whole command on one line.
This can even work on Linux installers, many of which leave a console running in the background – just load up a floppy/CD with import and the few libraries it requires for a first-rate run-anywhere screen grabber.