UAC stands for the University Admission Centre, this is the body that processes NSW University offers. NESA will send UAC your HSC results and they will use this to formulate an ATAR rank, this number is then used by universities to create scores/ cut offs for entry into their courses. University offers are then issued based on the preferences you provide and the ATAR ranking you have achieved.
If you are planning on attending university post school attaining and ATAR through UAC is the most common pathway.
To ensure you receive an offer to attend university you must
Pay the registration fee (early bird registration is a cost of $70)
Register your preferences - each preference is a specific course at a specific university - you may enter up to 5 preferences, you enter your preferences in order
If you are wanting to apply for university interstate or overseas you will need to apply using the relevant application body
Victoria - VTAC
Queensland - QTAC
Western Australia - TISC
South Australia - SATAC
Northern Territory - NTCET
Tasmania - TASC