Sources that digest, analyze, evaluate and interpret the information contained within primary sources. They tend to be argumentative. Sources that compile, analyze, and digest secondary sources. They tend to be factual. Secondary sources tend to come second in the publication cycle.
Tertiary sources tend to come last in the publication cycle.
Often scholarly periodicals and books. Often reference books. Newspaper articles, weekly news magazines, monthly magazines, diaries, correspondence, diplomatic records. Novels, poems, plays, diaries, correspondence.