Car
From the Bay Area you will want to take highway 80 heading to east to Sacramento. From Sacramento take 80 west. The exit to Davis is clearly marked by a large road sign.
Train
There is an Amtrak (http://www.amtrak.com/) stop within walking distance of campus, which serves the California Zephyr, Capitol Corridor, and Coast Starlight routes. Tickets from the Bay Area cost approximately $30 round trip. If you are flying into San Francisco Amtrak is the best way to get to Davis. From SFO take BART to Richmond. Exit BART and then take the stairs or elevator up to the Amtrak platform. The Richmond station is unstaffed so Amtrak tickets can be purchased on board the train for no additional fee.
By plane
Although UC Davis has its own small airport, it is really only possible to reach Davis by plane through commercial airports. The closest option is Sacramento International Airport SMF, from which one can then take a shuttle or taxi to Davis. A cheaper alternative is to take Yolobus. For $1.50 the #42 bus will take you to downtown Davis. You can then either utilize Unitrans, the local bus service or take a taxi to your destination. The airport is approximately a 30 minute drive from Davis.
The Oakland OAK and San Francisco SFO airports are also options for people who wish to visit the Bay Area as well as Davis. If your trip itinerary does not include the Bay Area, however, then SMF is the best option for commercial air travel to Davis. Amtrak trains are available from Oakland and San Francisco.
The Davis Airporter (http://www.davisairporter.com) is another option for people flying in commercially. While it provides service to your front door and is available at all hours of the day and night, it is considerably more expensive than public transportation. See also the Yolo Airporter (http://yoloairporter.com/...) and the Super Shuttle (http://www.supershuttle.c...).