A Primary Source is a historical document or artifact or person from the time period under study. This may include:
A Secondary source is a work written about an event or person after it has taken place. Some examples:
Peer review grows out of the 18th century Republic of Letters and the Royal Society, in which papers were submitted & reviewed by scholars before presentation and publication.
Now peer review is a process by which academic work is evaluated and improved before publication.
An article or book is submitted for review to an editor, who then sends it to 2 or 3 external reviewers, who in turn provide feedback and comments. They return the piece to the editor with notes, who then sends it back to the author. This process may repeat. This process improves the quality of a work, but also means publication takes Years after the work is written.