I'm planning a trip to San Fransisco this winter, and I'd like to know what kind of weather to expect there and what kind of winter clothes should I bring.
I'd appreciate your help.

hello coolorange,
In a "normal" winter, S.F. is RAINY!
As far as cold, it doesn't snow here, except for midnight flurries during a particularly cold rain storm that might dust the surrounding hills with a little white.
Coldest daytime lows will be upper 40's, coldest nighttime lows in the low forties. More likely, you'll get a mix of drizzle/rain with clear cool days in between, with daytime temps in the mid-fifties range. Bring a raincoat, preferably the kind with a liner, and good, water-shedding walking shoes. The coat should accommodate a sweater underneath. I personally never wear gloves, but it might be a good idea to bring them. To put it in a nutshell, it's difficult to die of exposure in S.F. weather, but it can be uncomfortable or inconvenient at times.

The one-word answer is: everything. Winter weather here often means rainstorms, temps in the 50's going down into the 40's at night (but almost never freezing or snow). So you want a good raincoat, with a hood, and probably an umbrella as well if you're going to do city activities.

On the other hand, when it's not raining, it can be in the high 60's, low 70's. So a few T-shirts or lighter weight shirts would not be a bad idea, especially if you can layer them with a sweater on top.

Layering is what we do here, all year really.

We don't really use winter clothes in SF in the sense of heavy wool clothing that is sometimes necessary back east--I have never, for example, worn my wool coat in SF. If you are thinking in terms of seasons, think fall but add rain gear. Also be prepared for the fact that it can rain/drizzle all day for a number of days (I think the record is 17 in an El Nino winter) and that it can also be sunny with highs in the 70's during the day (not unusual) dropping down to 40's at night.

About the winter coats - I didn't even own a winter coat until I bought one last winter for a trip to Denver. Then I got a chance to use it in Omaha last March. Now it is hanging in my closet waiting for my company to send me to some other snowy destination in the middle of winter. In the SF Bay Area, a rain coat over a sweater is all I've ever needed.

YOu people must be made of Iron cause I wear my winter coat in SF lots of times especially in the evening but then what is your definition of a winter coat. Mine is just a long wool coat and I do wear gloves. It does get cold and damp and into the 30's at night sometimes so the advice for dressing for Fall plus is good advice.

Nice wool scarf which my wife knitted.

A light, padded ski jacket with hood.

and some gloves for when it gets windy.

Wool coat? whazzat?