A primary source is a first person account by someone who experienced or witnessed an event. This original document has not been previously published or interpreted by anyone else.
A secondary source is one step removed from the primary original source. The author is reexamining, interpreting and forming conclusions based on the information that is conveyed in the primary source.
A tertiary source is further removed from the primary source. It leads the researcher to a secondary source, rather than to the primary source.