Southern California refers mostly to the attitude in Los Angeles more than the geographic region running from San Diego north up to Ventura along the Pacific Ocean. As opposed to the cultured people up in the Bay Area (they have the first rate wineries of Napa Valley,) Southern Californians lead a much more image conscious way of life.
Los Angeles is a city full of actors trying to make it working as waitresses and physical trainers. The set in West Hollywood are bold and beautiful and can be seen in the trendiest clubs sipping $14 cocktails sitting in plush velvet booths. Lawyers dart around the congested freeway in their BMWs flipping each other off, while many Mexicans ride the bus and hold three jobs all paying minimum wage. There are spots to go out in Santa Monica, down in Laguna Beach (surprisingly similar to the MTV show), and you can go and hope to spot a celeb on Sunset Boulevard. Los Angeles is alive with a buzz.
A few hours south of Los Angeles you have the much more chill San Diego with its surf beaches and a very Mexican feel. San Diego has great places to go out in Pacific Beach and the Gaslamp district downtown. Roadjunkies obviously love zoos and Balboa park is a great place to catch some free music or enjoy the art. The beach scene is friendly without being as fierce as LA.
Dreaded Tijuana is less than an hour to the south of San Diego with its cheap medicines, drug cartels and tequila induced debauchery.