You could say that Chinatown is a city within a city. It is a district with a large and dense Asian population; colorful facades, streetlamps, temples, restaurants and hundreds of shops that look like huge "99-cent stores".
The Chinatown district of San Francisco is the second largest Chinese community in the United States, after the one in New York.
The best way to get to Chinatown is probably through the large traditional Chinese gateway which you can find in the southern area of the district, in Grant Avenue close to Bush Street.
Once you are inside Chinatown, it is a good idea to visit Portsmouth Square. In this square the United States flag was first raised in the year 1846, and today you can see the current residents practicing tai chi or playing games like mah jong.
In order to get to know the area you could go towards Stockton Street where you can find food markets and watch the everyday life of the locals.
In Chinatown you can also visit the Chinese Cultural Center, although it only has an art shop and an art gallery where the works of some Chinese artists are displayed.
The best time to visit the Chinese district is midday, where all of the stores are open and the streets are full of locals. In the evening you can see lots of lights and colorful streetlamps, but the streets are quieter and many of the stores are closed.
If you find yourself in the Union Square area you can walk to Chinatown in a few minutes.
As strange as it may seem, the typical fortune cookies that are given out at Chinese restaurants were not invented in China; the first fortune cookies were actually baked in San Francisco.