A domain name is your 'address' on the internet. It is what your customers enter to find you in a web browser and will be the suffix of all your email addresses e.g. john @ mydomain . com.
The original commercial domain extension was '.com'. It is becoming harder to secure a .com domain name as many of them are in use or reserved by domain 'squatters'. A short .com domain name is easy to remember and can help your site be more easily recognised in advertising etc.
If your business mostly services Australian customers then a .com.au domain extension should be considered. To use this extension you must demonstrate a legitimate business connection to the name being sought, which prevents names being reserved by squatters for profiteering.