If you want to get from San Diego to Los Angeles (or vice versa) without renting a car and driving, the previous page describes flight options. If you want to keep your wheels on the ground instead, try a bus or train.
San Diego to Los Angeles by BusGreyhound runs from San Diego to Los Angeles several times a day. The fastest trip is a little more than 2 hours and fares are lower than flying.
San Diego to Los Angeles by Train
The Amtrak Pacific Surfliner takes about 2.5 hours to travel from San Diego's Santa Fe Depot to Union Station in Los Angeles, stopping in several Orange County cities (including Anaheim) on the way to downtown Los Angeles. From there, it goes on to Glendale, Burbank and the San Fernando Valley. Fares from downtown San Diego to downtown LA range from $35 to $50-plus.
Once you reach Orange County on Amtrak, you can transfer to Metrolink which runs several lines throughout the Los Angeles area. From Union Station, the LA Metro system trains go to many locations as well.
Here's a fun fact: Los Angeles' Union Station is known as the "Last of the Great Railway Stations" and in 1980 was placed on the National Register of Historic Places. If you enjoy watching movies or TV shows, chances are you will recognize the place from one of its' many appearances on the screen.