A primary source is generally defined as document or physical object which was written or created during the time under study. These sources were present during an experience or time period and offer an inside view of a particular event. Depending on your research topic and perspective, primary sources can include:
Secondary sources are those which are written ABOUT events in the past. They usually interpret those events through the lens of the time period in which they are written. New discoveries are made and attitudes change over time causing understandings of past events to change. Facts may remain consistent, but interpretations change, sometimes drastically.
NOTE: In the strictest sense, secondary sources are sometimes considered to be anything that refers to a primary source, such as a review of an artwork.