San Francisco does have a large Asian / Asian-American population. Historically (pre WWII), this was the primary entry point for most people from China and Japan, and the Chinese made up a huge part of the California labor pool in the 1800s (including largely supporting the gold rush and the building of the transcontinental railroad). Chinatowns aren't unique to San Francisco, but San Francisco's is, I think, considered one of the larger and more "authentic" (whatever that means) versions. There are still some families here that speak Chinese at home, although I think that's a very small population now.
San Francisco has one of--if not the--highest populations of Asian Americans in the country, almost 40% of the population.
