There is not one public transit company that serves all of Los Angeles; there are dozens. Trains may be Metro, MetroLink or Amtrak, depending on how far you're going. Buses from every smaller city and neighboring county all have buses that bring their residents into Los Angeles. Stand on a corner in Downtown LA and you will see buses from 10 companies stop at the same corner within a 15-minute time frame.
Here are the bus and train services that can get you from where you are to where you want to be in the LA area. Many of these have now been incorporated into Google Maps or Bing Maps, but they don't always give you the most logical route, so sometimes calling the service number for the specific company where you are departing will get you a more efficient route. Visit my LA Public Transit Guide for more tips on getting around LA with public transportation. Note: Some LA area bus systems do not operate on holidays.
Amtrak intercity and interstate train service.
Antelope Valley Transit Authority - Bus service in northern Los Angeles County
Burbank Local Transit - Serving the Burbank area with bus links to the Bob Hope Burbank Airport, the Burbank Metrolink Station and the North Hollywood Metro Red Line Station for travel to Hollywood and Downtown LA.
Carson Circuit - Multiple bus lines serving the City of Carson and linking to the Metro Blue Line.
Commerce (City of ) - Includes a couple weekday commuter routes, a Sunday church and shopping route, and the Citadel Express Route between Downtown LA and the Citadel Shopping Outlets.
Culver Citybus - Serves Culver City with connections to Venice Beach, Marina del Rey, Playa Vista, Westwood, Century City and LAX, connecting to the Metro Expo line in Culver City and the Metro Green Line near LAX.
El Monte Transit Services - Operates five routes within the city of El Monte. They also offer commuter shuttle service from the Metrolink Station and El Monte Bus Station to several business districts in the city.
Foothill Transit - Serves the San Gabriel and Pomona Valleys with 39 bus lines in 22 cities, covering an area that stretches from Downtown Los Angeles to southwest San Bernardino County.
Glendale Bee Line - Seven bus lines serving the city of Glendale and linking it to La Crescenta to the north and Burbank to the west, with connections to Foothill Transit and Metrolink rail connections to other cities.
Golden Empire Transit (GET) in the Bakersfield area.
Long Beach Transit - Services extend beyond Long Beach to Seal Beach and Los Alamitos in Orange County, as well as to the nearby cities of Hawaiian Gardens, Cerritos, Lakewood, Bellflower, Paramount, Carson, Compton and Dominguez Hills. Buses connect to the Metro Blue Line at multiple locations. Long Beach Transit also operates two water taxi services in summer.
Things to Do in Long Beach
LADOT - City of Los Angeles Department of Transportation has local and express buses that serve every corner of the city of Los Angeles, plus many neighboring cities from the beaches to the valleys.
LA County Metropolitan Transportation Authority (Metro) - Manages the Metro rail line service as well as its own bus lines that overlap and connect other public transit services in the county and connect to Metrolink trains and neighboring counties.
Read my article on How to Ride the LA Metro
Metrolink Trains - Limited-stop commuter trains between urban centers in Southern California.
Montebello Bus Lines - Connects Montebello to East Los Angeles, Downtown LA, San Gabriel and Alhambra to the north, and Whittier, South Gate and La Mirada to the south.
Norwalk Transit - Serves Norwalk and adjacent communities of Artesia, Bellflower, Cerritos, La Mirada, Santa Fe Springs, Whittier, and unincorporated area of Los Angeles County, overlapping and connecting with Long Beach Transit and Metro buses, and connecting to the Metro Green Line in Norwalk.
Orange County Transportation Authority (OCTA) - Offers 65 routes serving all of Orange County, with some lines reaching across the county lines to LA and San Diego County. OCTA also manages Metrolink services in Orange County.
Things to Do in Orange County
Santa Clarita Transit - Serves the city of Santa Clarita in north Los Angeles County and connects it to Downtown Los Angeles, the North Hollywood Metro Red Line Station, Century City and UCLA, and other San Fernando Valley cities.
Santa Monica Big Blue Bus - Serves Santa Monica and has lines that run into various parts of Los Angeles including Pacific Palisades, Venice Beach, Downtown LA, Koreatown, Culver City, Century City, LAX, and the Metro Green Line Aviation Station.
Things to Do in Santa Monica