There are three types of sources: primary, secondary, and tertiary.
Primary sources are original first hand materials which have not been interpreted. These include materials like diaries, court proceedings, institutional records, statistical data (raw data), or speeches. Original scientific research is also a primary source. See the Primary Sources research guide for information on locating primary sources.
Secondary sources are materials that evaluate, interpret, and discuss primary sources. Scholarly journal articles, histories, and monographs are all secondary interpretations of original materials. Start with some database recommendations in the box to the right (Proquest, etc.) Here's one more: Taylor and Francis.
Tertiary sources are materials like encyclopedias, Wikipedia, or other reference works. These works compile and summarize primary and secondary sources. Consult these sources during the initial stages, but use primary and secondary sources for your research. Access reference databases here.