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. (TIP: If the paper has a "Methods" section it's probably original scientific research.)
Secondary sources are materials that evaluate, interperet, and discuss primary sources. Scholarly journal articles, histories, and monographs are all secondary interpretations of original materials.
Tertiary sources are materials like encyclopedias, Wikipedia, or other reference works. These works compile and summarize primary and secondary sources. While it is appropriate to consult these sources during the initial stages you will want to find primary and secondary sources for your research.