Most desktop environments come with a built–in screenshot facility. Try pressing the Print Screen button (often labeled with an abbreviation like “PrtScn”), and you might see the screen blink, hear a camera shutter sound, or get some other indication that a screenshot file is available. If this works other variants might be available:
Alt–Print Screen to capture only the currently–focused application
Shift–Print Screen to capture a custom rectangle of the screen
Ubuntu saves screenshots in the ~/Pictures directory.
This and many other tools offer convenient features beyond keyboard shortcuts:
gnome-screenshot --interactiveopens a window to set up a screenshot.
gnome-screenshot --include-pointerincludes the mouse pointer in the image.
gnome-screenshot --delay=5waits for five seconds before taking the screenshot. This allows you to capture any process which would be broken by pressing a key, or to easily capture screenshots at an interval.
--file=FILEallows you to set a custom filename, which you could, for example, use in a loop to create sequential image files: