Reference books include encyclopedias, dictionaries, manuals, and handbooks. They contain introductory material, or provide standard data and facts about a topic. In biology, protocol manuals for conducting standard lab procedures are also becoming an important reference source.
A few of the most critical reference sources are listed here. Several more are available in the reference section and in the book stacks.
Wikipedia is often a great source of background information. The worry by some librarians and professors is that there is little control over changes made to entries, so some of the content can easily be erroneous. That said, Wikipedia can be still be a useful tool -- just be sure to verify the information you find there! In good Wikipedia entries, the suggestions for further reading and reference can be particularly useful.
Check the Wikipedia Biology Portal.