Now: A lot of history remains preserved (or ignored) in this area - many old hotel and apartment buildings (some dating back to the years just after the 1906 earthquake) have survived without makeover or gentrification. Restaurant gems, ethnic offerings unlike any other place in the city, are sprinkled throughout. Just ignore the people hanging around the seedier intersections and enjoy the faded beauty.
Then: The name "Tenderloin", according to some accounts, refers to the cuts of meat that SF police officers used to buy with the extra pay they would get for working the neighborhood.
How To Get There: Neighboring Union Square, you can just walk west on any street near there, and you'll find yourself in the Tenderloin within a couple of blocks. You can also walk up from Civic Center.

Jones Street at O'Farrell, in the Tenderloin

Intersection of Geary Street and Jones

Jones Street at Geary

Dottie's True Blue Cafe, on Jones Street between Geary and O'Farrell Streets

Morning line to get into Dottie's

Nile Cafe, on Jones Street between Geary and O'Farrell

Nazareth Hotel, on Jones Street at Geary Street

Looking east on Geary Street

Boeddeker Park, at the corner of Eddy and Jones Streets

Eddy Street and the Elm Hotel, in the Tenderloin

Apartment buildings in the Tenderloin

Hospitality House, on Turk Street

Lahore Karahi Indian and Pakistani Restaurant, on O'Farrell Street near Leavenworth Street

Hotel wall advertisements in the Tenderloin

Original Joe's Italian Restaurant and Steakhouse, on Taylor Street

Glide Memorial Methodist Church, on Taylor at Ellis Street

Ambassador Bar on Geary Street at Leavenworth Street

Rye Bar, on Geary Street at Leavenworth Street

Kayo Books, on Post Street at Leavenworth Street

Cafe Royale, on Post Street at Leavenworth Street

Cairo Nights, on Geary Street near Larkin Street

A La Turca Turkish Cuisine, on Geary Street near Larkin Street

Geary Street at Larkin Street

Moulin Breakfast Cafe, on Geary Street at Larkin Street

Larkin Street at Post Street

Cafe Bean, on Sutter Street at Jones Street

Commodore Hotel, on Sutter Street at Jones Street