Generally speaking, a primary source is a document that was created at or during the time being researched. Secondary sources are ones that are created after the event and usually are reviewing or analyzing the event or period.
For example, a diary of a Civil War soldier written during the war would be considered a primary source. However, a book or article written in the 1990s analyzing a particular battle in the war would be considered a secondary source.
Primary documents are an important part of historical research. Finding these documents can be a little tricky sometimes, so here are few ideas to help you find some primary sources, especially older primary sources.
Here are some websites that contain some online historical primary sources.
Here are just a few electronic books that contain primary source material.