Often when we're studying new topics and issues, we need to get a little bit of background information to help us understand what we're studying. Reference sources are very useful for this kind of background information.
Reference sources include encyclopedias, atlases, bibliographies, almanacs, and dictionaries. They allow you to look up a topic and find basic information, and sometimes citations and references for further, more in-depth reading.
There are a few reference sources that might prove useful for your assignments. You don't usually want to cite reference material in your papers, but they are very useful for getting a better understanding of your topic and a sense of where to look next.
If you want to find some contemporary analyses of the time period and movements you're studying, article databases and indexes are what you need. These sources can let you know what experts think about the topics you're studying.