This is a excellent, classic natural history museum with terrific wildlife dioramas, a gem & mineral hall, Meso-American artifacts, and dinosaur fossils. There are several exhibits that reflect Southern California, particularly Becoming Los Angeles on the development of the area and the Age of Mammals, which features some of the prehistoric animals of L.A.
While not as huge as the major natural history museums of Chicago or New York, it's still quite big, and you can easily spend several hours wandering its halls. It's a popular museum, so expect crowds (especially families) on weekends or during school breaks. There is parking in the garage off Figueroa St, and it's really close to the Expo/USC station on the Metro E Line.
Overall, this is a really great museum featuring lots of local history, and it's well worth a visit.