Austin Nightline
Certainly the most popular and well known area for nightlife is the East 6th Street District. A rival, but companion, district is now flourishing on the west side of Congress Avenue and is known as the Warehouse District (the crowds here are a bit olde and less rowdy). These two areas have the largest concentration of nightclubs in the Austin area.
That is not to say that there aren't other spots out there, especially if you lean toward country or tejano. The links below will assist you in finding a venue that suits you if want to head out on the town.
Just a word of caution, though. APD has no tolerance for DUI/DWI....so drink responsibly and take along a designated driver, if necessary.
East 6th Street Entertainment District (PDF file - map)
West 6th Street Warehouse District (PDF file - map)
Live Music Venues and Nightclubs