The oldest Chinatown in North America, the largest Chinese population outside Asia - and a bold break from the foggy pastels of San Francisco... epitomized by the colorful Chinese New Years parade
Chinatown is a great place to stock up on fruits, vegetables, exotic herbs, swords, and as many trinkets as your heart desires. And a great opportunity to try chicken (or goose) feet :)
Visitors are sometimes afraid of Chinatown - and it's good to be careful with your wallet anywhere you go - but locals are usually more concerned about Muni slowdowns (especially on the 30 line) than theft.
For those seeking a meal, the R & G Lounge is a favorite.
Stockton is the primary commercial street with lots of fruit and vegetables spilling onto the sidewalk, while Grant is more iconic for many visitors (with lots of shops). The alleys are worth a gander as well - Ross Alley is purportedly the oldest in SF, and home to the Fortune Cookie Factory (!)
Wikipedia: http://en.wikipedia.org/wiki/Chinatown,_San_Francisco,_California