fish is a user friendly command line shell for UNIX-like operating systems such as Linux.
Bash is a standard (de facto), as every *nix user now but... this is the beauty of the OpenSource: any other programmer can think "hey, I want to remake this in another way!" and, than, do!
Fish is a beautiful example.
For screenshots-addicted users (like me ;)), go here.
-EDIT- Thanks to Nemo for "errata-corrige".