They contain more than 30 million names of people who registered to vote from 1900 to 1944 (and through 1968 for a few counties). Their frequency makes them great for filling gaps between federal censuses.
Search by name and county, then click on a name in the results to see a digitized images of the registration books page showing that person. You’ll see his or her name, occupation, address and party affiliation. Some registers show an age, and early ones may give naturalization details. This one is from 1916.
Women received the right to vote in 1911 in California, so you won’t see them in the Great Registers until 1912.
Ancestry.com’s images came from the collections of the California State Library in Sacramento—where you can access the lists from 1866 to 1898, too. Local libraries and genealogical societies in California often have Great Registers for their areas, and many counties’ lists are on Family History Library microfilm (you can borrow it though your local Family History Center).