Even if you are already familiar with your topic, in the course of your research you'll likely come across some related concepts or associated individuals/organizations that are new to you. Encyclopedias (including Wikipedia!) are great places to gather background information.
Note: while these are useful resources, your research shouldn't stop here: use the information you find here to conduct searches for scholarly, popular, and government sources (see the menu on the left for tips on locating those sources). Those sources should form the bulk of your research. If the information you find in encyclopedias and other reference sources influences your final product, be sure to cite it -- but don't rely solely on reference works to support your research.