Scientific literature is divided into two basic categories - "primary" and "secondary".
A Primary Source is a document or physical object which was written or created during the time under study and is the result of original scientific research or observation. Some types of primary sources include:
SCHOLARLY JOURNAL ARTICLES: An article reporting new and original research or findings written by the original researcher.
ORIGINAL DOCUMENTS: Diaries, speeches, manuscripts, letters, interviews, news film footage, autobiographies, official records
CREATIVE WORKS: Poetry, drama, novels, music, art
RELICS OR ARTIFACTS: Pottery, furniture, clothing, buildings
A Secondary Source interprets and analyzes primary sources. These sources are one or more steps removed from the event. Secondary sources may have pictures, quotes or graphics of primary sources in them. Some types of seconday sources include:
MAGAZINE/JOURNAL ARTICLES - These are articles which interpret or review previous findings or present findings in way more accessible to the general public. They are not written by the original researcher. Examples would be Scientific American or Psychology Today.