As mentioned in my introduction, the first Native Americans to inhabit San Francisco were the Ohlones around 10,000 years ago. They were attracted by the vast natural resources that were available for them to hunt and to settle their villages and chiefdomes. The first Europeans, a Spanish named Juan Rodriguez Cabrillo heading north for their voyage and later on in 1579, an Englishman named Sir Frances Drake failed to find the entrance to San Francisco bay.
It was not until the late 1760 and early 1770 that Spanish voyagers named Don Gaspar de Portolà and Fra. Junipero Serra found the region as a potential military and religious settlement. They built the Presidio Army Base and the Catholic church captured and enslaved the Native American Ohlones.
Russian fur-traders also settled in the area and this is where the Russian Hills name came from.
After its freedom from Spain in the 1820's, it became part of Mexico. About a decade after, an Englishman named William Richardson built the first homestead and expanded to a town they named Yerba Buena, after the discovery of the plants abundance around the area. In the 1840's, Yerba Buena's population doubled after Mormon's arrived in their ship. After California was claimed by the United States, Yerba Buena was claimed two days after and it's name officialy changed to San Francisco in 1847.
The California gold rush starting in 1848 led to a large boom in population, including considerable immigration. Between January 1848 and December 1849, the population of San Francisco increased from 1,000 to 25,000. The rapid growth continued through the 1850s and under the influence of the 1859 Comstock Lode silver discovery.
The population boom included many workers from China who came to work in the gold mines and later on the Transcontinental Railroad. The Chinatown district of the city became and is still one of the largest in the country; today as a result of that legacy, the city as a whole is roughly one-fifth Chinese, one of the largest concentrations outside of China
It was during the 1860s to the 1880s when San Francisco began to transform into a major city, starting with massive expansion in all directions, creating new neighborhoods.
On April 18, 1906, a devastating earthquake centered immediately offshore of San Francisco. The quake had a magnitude of 7.8 on the Richter scale. Water mains ruptured throughout San Francisco, and the fires that followed burned out of control for days, destroying approximately 80% of the city, including almost all of the downtown core. The official death toll at the time was 478, although it was officially revised in 2005 to 3,000+.
During World War II, San Francisco was the major mainland supply point and port of embarkation for the war in the Pacific. It also saw the largest and oldest enclave of Japanese outside of Japan, Japantown, completely empty out many of its residents as a result of Executive Order 9066 that forced all Japanese of birth or decent in the United States to be interned. By 1943 many large sections of the neighborhood remained vacant due to the forced internment. The void was quickly filled by thousands of African Americans who had left the South to find wartime industrial jobs in California as part of the Great Migration. Many African Americans also settled in the Fillmore District and most notably near the Bayview-Hunters Point shipyards, working in the dry-docks there. After World War II, many American military personnel who fell in love with the city when they left for or returning from the Pacific, settled in the city, prompting the creation of the Sunset District, Visitacion Valley, and the total build out of San Francisco.
San Francisco's frontier spirit and wild and ribald character started its reputation as a gay mecca in the first half of the 20th century. World War II saw a jump in the gay population when the US military actively sought out and dishonorably discharged homosexuals. From 1941 to 1945, more than 9,000 gay servicemen and women were discharged, and many were processed out in San Francisco.
The late 1960s also brought in a new wave of lesbians and gays who were more radical and less mainstream and who had flocked to San Francisco not only for its gay-friendly reputation, but for its reputation as a radical, left-wing center. These new residents were the prime movers of Gay Liberation and often lived communally, buying decrepit Victorians in the Haight and fixing them up. When drugs and violence began to become a serious problem in the Haight, many lesbians and gays simply moved "over the hill" to the Castro replacing Irish-Americans who had moved to the more affluent and culturally homogeneous suburbs. The Castro became known as a Gay Mecca, and its gay population swelled as significant numbers of gay people moved to San Francisco in the 1970s and 1980s.
During the dot-com boom of the late 1990s, large numbers of entrepreneurs and computer software professionals moved into the city, followed by marketing and sales professionals, and changed the social landscape as once poorer neighborhoods became gentrified. In 2001, the markets crashed, the boom ended, and many left San Francisco. By 2003, the city's economy had recovered from the dot-com crash thanks to a resurgent international tourist industry and the Web 2.0 boom that saw the creation of many new internet and software start-up companies in the city, attracting white-collar workers to recent University graduate young adults from all over the world.
Resources:
http://www.sanfrancisco.com/history/
http://en.wikipedia.org/wiki/History_of_San_Francisco
San Francisco's climate can be compared to that of a cool-summer Mediterranean. It is famous for its cool winds and fog. Temperatures exceed 75 °F (24 °C) on average only 28 days a year, which makes San Francisco weather on the cool side all year round. The dry period of May to October is mild to warm, with average high temperatures of 64–71 °F (18–22 °C) and lows of 51–56 °F (11–13 °C). The rainy period of November to April is slightly cooler, with high temperatures of 58–64 °F (14–18 °C) and lows of 46–51 °F (8–11 °C). On average, there are 67 rainy days a year, and annual precipitation averages 23.8 inches (604.52 mm). Snow is extraordinarily rare, with only 10 instances recorded since 1852, most recently in 1976. 
Megalopolis was coined by Gottman as a very large functional urban region that provides the whole of America with so many essential services, of the sort a community used to obtain in its 'downtown' section, that it may well deserve the nickname of 'Main Street of the nation.' According to Gottman, the California coast actually has a megalopolis called SanSan that spans from the San Francisco Bay area to San Diego. It is about 500 miles long. 
The California gold rush starting in 1848 led to a large boom in population, including considerable immigration. Between January 1848 and December 1849, the population of San Francisco increased from 1,000 to 25,000. The rapid growth continued through the 1850s and under the influence of the 1859 Comstock Lode silver discovery.
San Francisco's frontier spirit and wild and ribald character started its reputation as a gay mecca in the first half of the 20th century. World War II saw a jump in the gay population when the US military actively sought out and dishonorably discharged homosexuals. From 1941 to 1945, more than 9,000 gay servicemen and women were discharged, and many were processed out in San Francisco. The late 1960s also brought in a new wave of lesbians and gays who were more radical and less mainstream and who had flocked to San Francisco not only for its gay-friendly reputation, but for its reputation as a radical, left-wing center. These new residents were the prime movers of Gay Liberation and often lived communally, buying decrepit Victorians in the Haight and fixing them up. When drugs and violence began to become a serious problem in the Haight, many lesbians and gays simply moved "over the hill" to the Castro replacing Irish-Americans who had moved to the more affluent and culturally homogeneous suburbs. The Castro became known as a Gay Mecca, and its gay population swelled as significant numbers of gay people moved to San Francisco in the 1970s and 1980s.