The Transit Visa (771) is for people who need to transit through Australia on their way to another country for a period of no longer than 72 hours. It is also for people who need to transit through Australia in order to join a ship as the member of a crew.
This is a temporary visa and must be applied for from outside of Australia.
You will need to provide evidence of your travel arrangements when applying for this visa.
This visa does not allow work or study and can not be extended.
If you are going to be in Australia for longer than 72 hours then you will need to apply for another visa type.
For citizens of most countries, if you are transiting through Australia and will not leave the airport transit lounge, then you do not need to apply for this visa.