Tijuana is the largest city in Baja California. It is located in the northwestern corner of Mexico, right south of the US-Mexico border. It is very easy to get to from southern California, in fact it is a short 20 minute drive from downtown San Diego down the 5 freeway. Crossing into Mexico at the San Ysidro border is easy, it's getting back into the US that's a pain. You can drive accross, or park on the US side, and walk, which would be recommended for a day trip if you want to just stay in the area. The busiest border crossing in the world, it can sometimes take hours of waiting to cross back into the US, although for US citizens, all that's needed to go back and forth is a valid ID, provided you don't go to far south into Mexico. (Update: not true anymore. Now even US citizens need either a passport, a passport card, or a government ID and proof of citizenship (birth certificate); even for a day trip across the border.
Getting around in Tijuana (or TJ as it is called by some in Southern California) is fairly easy. You can walk to many locations from the border crossing, but be ready to be hassled by vendors and children alike. There are cabs all over, but be sure you know where you want to go because they may be ready to take advantage of you and your American dollars.
Tijuana is a major city, and in addition to it's reputation of catering to Americans looking for a good time (many people under 21 go there to drink from the States), it also holds its own as a major city. Its proximity to the US attracts tourist dollars, people from other parts of Mexico, and businesses looking for a good location with cheap labor. There is a good sized middle class, but there is also a seedy side to the city, so be sure to use good judgement in order to stay safe.