A centroid of night clubs and home to the Folsom Street Fair, SOMA is the hard edge of San Francisco. Adjoining the Civic Center, Downtown, South Beach and Potrero Hill, the area is large, industrial (dot-com boom HQ) and best experienced with a game plan.
11th st and Folsom is the main center of clubbing activity, but 1015 Folsom (SF's biggest and most famous night club) and the Endup are towards Downtown off 6th St.
Wikipedia: http://en.wikipedia.org/wiki/South_of_Market