recordmydesktop is a powerful screencasting commandline tool for Linux. To capture the screen:

$ recordmydesktop

Press Ctrl+C to quit capturing. The captured video will be encoded in Ogg Theora format and saved as out.ogv (if out.ogv exists, as out-1.ogv and so on). To save the file in another name, say myout.ogv:

$ recordmydesktop myout.ogv

There is also a GTK frontend for recordmydesktop. To launch it:

$ gtk-recordMyDesktop