Primary sources present first-hand accounts or direct evidence. They are created by witnesses or recorders who experienced the events or conditions being documented, and can also include autobiographies, memoirs, and oral histories recorded later.
Secondary sources interpret and analyze primary sources. Because they are often written long afterward by parties not directly involved (but who may have special expertise), they can provide historical context or critical perspectives. Secondary sources can include pictures, quotes, or reproductions of primary sources.
Depending on the subject (or your purpose in using the source), sources can fall into both categories. For example, an art critic's review of an exhibition opening is a primary source, because it is commenting directly on a current event; whereas an article discussing an artist's body of works which includes information about that exhibition would be considered a secondary source as it is after the fact.
Credit for definitions: Yale University Library, Art History LibGuide
This short video will give you an overview of what primary sources are and how they are different than secondary sources.