A primary source is a document or object which is contemporary to the period you are studying; that is to say, a primary source was written or created during the period under study.
Primary sources can come in many forms. Written documents might include diaries, letters, newspapers, books, church records, government records, census records, video or audio recordings of an event, and autobiographies, among others. Music, poetry, novels, and plays written during the period are also primary sources. Finally, material items might also act as primary sources; historians use clothing, furniture, architecture, and pottery as primary sources.
Because primary sources were created during the time you are studying, and because they were created by a person who witnessed that event, it is important to remember that they contain bias, or the creator's particular viewpoint. Primary sources must be read and used in context; that is, it is important to know the author's personal background and motivation so you can interpret the meaning and value of the information in the source.