The Tenderloin is San Francisco's neighborhood that is most widely recognized for its seediness. The Tenderloin is home to many of the city's strip clubs and bars, as well as plenty of drug users, prostitutes and the homeless. It is supposedly named after the fine cuts of meat that the police officers working the area could afford. Rent here is lower than pretty much anywhere else in the city. For a reason.
On many maps of the city (and on Craigslist), the area is simply titled "downtown." Its borders are rather ambiguous, but it lies north of Market and generally ends at Van Ness, Taylor and Geary. The Tenderloin contains several subsections that go by different names, such as Polk Gulch (the length of Polk Street between Broadway and Eddy), Civic Center (for nearby Civic Center Plaza) and the Theater District (obvious enough). The area bordering Nob Hill is often referred to as "lower Nob Hill" and sometimes "the TenderNob." (If you see an apartment listing on Craigslist saying "Not Tenderloin", it's probably in the TenderNob.)