The very basic of what is needed for a website is an address. It is called a domain name and is much like an street address.

One cannot 'buy' domain names they can only be licensed for varying terms.

In Australian the .com.au (dot com dot au) names have a 2 year minimum and a 10 year max whilst the most popular .com (dot com) names can be licensed for a minimum of 1 year. The Australian names MUST be the mostly the same as the name you have a business registration and in some cases you have to get special permission to register the name. The dot com names can be registered by anyone anywhere.

The dot com names are the most popular due to many factors, not the least, the shortened address which has to be used for printing and entering into the browser when surfing the internet.

Many Australian businesses use both extensions in case someone with the same name registers the other address and causes confusion.