Hi! I can not make a clear responce! For me the different shells have the same purpose. I think the difference is mainly a question of syntax and functions. if you want to investigate you can take a look at this book The UNIX CD Bookshelf, 2nd Edition (w/ CD-ROM) http://www.amazon.com/exec/obidos/ASIN/0596000006/107-3425371-9934929 the main shells are describe inside!
A shell is a command interface that is used to interperate and process you command. In my opinoin, the difference between shells is as follows: Bourne - Simple and most compact shell, better for Scripting KORN - A superset of the Bourne shell that allows you to edit the command line, also good for scripting. C shell - Outdated, but good for user interactive use and job control. I would reccomend the KORN shell, it has the most features and can do almost everything that the other shells can do.