Shells are a user-interface for your operating system's services. In windows, the shell is explorer.exe, in linux, it is generally console-based and you have several choices (sh, tcsh, ksh, bash, zsh, ... ).
|sh||The original shellscript. Limited, but targeting shellscripts to sh ensures they will run just about anywhere.|
|bash||The shell you'll find pre-installed on most servers|
|zsh||Backwards compatible with bash, introducing some of the features of ksh, and providing several very dynamic completion/prompt features. zsh is awesome.|
|xonsh||a python based shell. very cool.|