From the State Department's web site (linked below):
ENTRY / EXIT REQUIREMENTS FOR U.S. CITIZENS: You must have a valid U.S. passport and a visa to enter Australia. Most U.S. passport holders traveling to Australia for tourism or business purposes for less than 90 days can obtain an Electronic Travel Authority (ETA). The ETA is an electronic label-free visa and can be obtained at the ETA website for a small service fee. Airlines and many travel agents in the United States are also able to apply for ETAs on behalf of travelers. If you overstay your ETA or any other visa, even for short periods, you may be subject to exclusion, detention, and removal by the Australian Department of Immigration and Citizenship (DIAC). You can find more information about the ETA, other visas, and entry requirements from the Embassy of Australia at 1601 Massachusetts Avenue, NW, Washington, DC 20036, via the Australian Visa Information Service at 905-280-1437 (toll charges to Canada apply) or their website.