Domain Names
A domain name is the bit after the "@" in an email address and the bit before the "www" in a web address - e.g. bigpond.com, iinet.net.au, ninemsn.com.au, yahoo7.com.au, google.com etc.
There are many different kinds of domain names but they do fall in to categories. The only two that are really important for your business are domain names that end in .com.au &/or .com From experience, domain names that end in .net(.au), .info, .org, .tv etc are not really worth it, because people tend to type .com(.au) anyway. The only reason to register variants in our opinion is to ensure that nobody else can.
The difference between .com and .com.au is price and instantly identifying yourself as Australian. "Dot com" domain names don't have any rules at all but tend to be thought of as from the USA. "Dot com.au" domain names, on the other hand, need to be tied to an Australian business. "Dot" com domain names vary between US$6.95 and AU$40 per year to own whilst "Dot" com.au domain names vary between about $30 and $140 per 2 years.
Domain names appear to have something to do with SEO too. So if you sell apples, and if you could get it, but names like "sydneyapples.com.au", for example.
Generally we recommend you buy a ".com.au" domain name. My friends over at ireckon are very good and ultra-cheap - check them out!

