Yad Vashem is actually a complex of museums and memorials. It's also an education facility housing some of the most important archives. I recommend scheduling at least half a day to fully take in all that this site has to offer. You can reach Yad Vashem by car, taxi, or several bus lines from the Central Bus Station in Jerusalem.
Two of the most noted areas of Yad Vashem are the Garden of the Righteous and the Hall of Remembrance. The Garden of the Righteous is where non-Jews who assisted Jews during the Holocaust are honored. A tree has been planted here for Emilie and Oskar Schindler.
The Hall of Remembrance is where memorial services are held. It is a dark building, with a central eternal flame that illuminates the names of the death camps. Victims' ashes are also located here. This is a very solemn place and silence or quiet talking should be practiced.
Free tours are available in English once a day for most of the week. For the individual visitor, there is a 2-hour walking tour of Yad Vashem that allows you to visit all the major sites along a path. You can also follow a longer 3- to 4-hour walking tour, or take a car tour.