These days, more people know it as the CommandLine.

Traditionally, the main program that is used for interacting with a computer, for example for running other programs.

"Real" Unixes simply have "sh", which is the Bourne shell (or a variant). (?? - is this correct?).

There are now many "sh-compatible" shells, which use similar syntax. The shell used on GNU/Linux systems is bash(1), but others include ash(1), ksh(1)? and zsh(1). ksh is quite common on systems such as Solaris but it (and the original Bourne shell) have licences that don't meet the definition of Free software.

See BashNotes for hints and examples for using bash.

Another common shell (with different syntax than sh-compatible shells) is csh(1) (the c-shell.... get it?), and its Free counterpart tcsh(1). The BSD family use csh as the default shell for some reason.

See CshProgrammingConsideredHarmful for why you shouldn't use this.