A shell is a program which interacts directly with an operating system such as MS-DOS, Unix/Linux, Mac OS X, or others — but it is most commonly associated with Unices. It is often referred to as a 'console' or 'command line', because it is controlled using typed commands rather than mouse or graphical interface input.

Most often, when interacting with a remote computer (as one would when configuring WordPress), an additional "faux" shell is involved called SSH.

Some popular shell programs are:

  • Bash (Bourne Again Shell)
  • Tcsh (an expanded C Shell)
  • Zsh

Reference http://codex.wordpress.org/Glossary