A domain name is an address on the internet. For example, your business may be called Main Road Book Store. You could register a domain name such as mainroadbooks.com.au.
A domain name on it's own however does very little other than preventing someone else from registering the same domain name.
To make use of your domain name you need to combine it wih a web hosting or email only package to make use of web (http://www.mainroadbooks.com.au) and email (email@mainroadbooks.com.au)