What are primary and secondary sources and how do historians use them when researching different events in history?

A primary source is a person (or a record produced by a person) who was actually there, as a witness or a participant of the event in question. A secondary source is someone who obtained information from a primary source and who then wrote or spoke about it. Primary sources are the more authoritative, however they are not always available, and sometimes you have to settle for lesser sources.