The NXT Shell is a program that is run on the NXT brick in the background during direct mode execution. Since some remote mode commands do not have a direct mode analog, the shell will execute these commands. The shell has 2 main purposes: 1) the sensor view screen and 2) execution of shell commands.
http://zone.ni.com/reference/en-XX/help/372962A-01/lvnxt/nxtshell/
This is an important requirement for running programs from a computer for debugging/etc.
