An ABN (Australian Business Number) is a unique 11 digit number issued by the ATO to all eligible entities. You must have an ACN before you can apply for an ABN. A company's ABN consists of their ACN with a 2 digit prefix. The ABN system works together with the GST and PAYG withholding system.
As a general rule of thumb, your ACN identifies your company, and an ABN identifies your company to other businesses and various government agencies (such as the ATO).