Are you referring to the shell?
BASH is the most common standard as far as shell's go. There are many other shells that can be used, however. While the wikipedia entry on the subject is not exactly the most thorough, it seems to provide a good overview.
http://en.wikipedia.org/wiki/Unix_shellAs far as Linux goes, generally what you get with the distro "ought" to be what you use most often. This doesn't mean you cannot use another shell- far from it- but most distros contain a number of shell scripts for various purposes, and these will of course not always work in other shell implementations. Whether this constitutes a problem depends on the specific user and what they are trying to do, of course.
