Who qualifies as a peer?
Any expert in the field of study can be considered a peer. They are usually either professionals or academic professors.
What do they review?
In addition to reading the article for comprehension, reviewers will evaluate content including whether data is consistent with summaries and arguments. They will make notes of any bias or "hidden agendas." They will also check reference entries for quality and findability.
How long does it take?
The whole process can take several months, and many articles will go through a double blind review - two different sets of evaluations! This ensures the information has been thoroughly checked for reliability.
Image taken from Understanding Science