Burritos are essentially a wrap, of Mexican-American origin, containing some combination of meat, beans, rice, cheese, salsa, sour cream, and guacamole. The best burritos are to be found in the Mission District, San Francisco — home of the famous Mission Burrito. Mexican restaurants serving up burritos in this neighbourhood tend to use the word Taqueria.
Taquerias are almost always cash-only. I tried quite a few different places while living in San Francisco for a summer.
El Farolito is hands down the best Taqueria in San Francisco. It is located on the east side of Mission street immediately north of the 24th St BART station plaza. Crucially, they do not skimp on the meat.
Whenever I visited, it was rather busy with a line out the door (or close to it). It is always difficult to get a spot inside to sit — try to scout out a spot either while in line or while waiting for your food to be ready.
Honorable mention, Cancun is firmly #2 on my list. A great spot if you are at the other end of the Mission District since it is closer to the 16th St BART than El Farolito.