E-mail
With e-mail the domain is the part after the @ sign.(E.g. In the e-mail address you@yourname.com, yourname.com is the domain)

Web Sites
You can use your domain to point customers towards your web pages when they surf the Internet.
So www.yourname.com would take them to your web site.

Do you need a domain
If you would like your own e-mail address that you can keep forever, then yes. Forget ever having to e-mail people to tell them that you have had to change your e-mail address again!

If you need an easy-to-remember e-mail address, then yes.
jane@janesplace.com is easier than janeb034@hotmail.com

A good domain for a web site is essential if you want to be easy to remember. Most search engines also use the domain name to help direct people to the most suitable web sites.
http://freespace.yourisp.net/mike.davis/index.html is not quite as catchy at www.mikedavis.co.uk