UC Library Search is the University of California’s unified discovery and borrowing system. Use it to find books, articles, media, archival collections, and more across most UCB libraries (notable exception: Berkeley Law Library), as well as the collections of other UC libraries.
See the UC Library Search user guide.
Here are other guides to look at when you want library resources specific to your major or subject area - for example, recommendations for article databases are listed in the 'articles' tabs. The guides are geared towards grad students and others doing research in these subject areas, so there's a lot of information.
And a couple more: