Secondary sources interpret, analyze, and discuss the evidence provided by primary sources. Examples of secondary sources include scholarly books and articles.
Secondary sources are generally a second-hand account or observation at least one step removed from the event, i.e., accounts written after the fact by people not present when an event took place. Such sources are second-hand interpretations of what occurred.
Secondary sources, however, can be considered to be primary sources depending on the context of their use. For example, Ken Burns' documentary of the Civil War is a secondary source for Civil War researchers (because it consists of Burns' interpretation of primary source materials from the Civil War), but a primary source for those studying documentary filmmaking.
Secondary sources benefit from the filter of time and differing cultural contexts and perspectives which may assist (or interfere with) scholarly analysis.
Secondary sources can include:
commentaries and critical reviews;
books other than fiction or autobiographies
journal, newspaper, and magazine articles written well after an event takes place