They can provide a very powerful addition to the web developers toolkit for validating form data such as Credit Card Numbers, Order Numbers etc, or for removing any 'odd' characters from a user entered parameter before making it part of a SQL query, or even for updating a path on every page in your website.
There is certainly a lot more detail into which you can with Regular Expressions in Javascript, let alone any other language, but I hope that this tutorial has covered enough to get you started, try the resources listed below for more information.
Resources
I recommend the following resources on Regular Expressions, although I am sure that there are many more (as a quick search on Google proves) :