What is considered a primary source varies somewhat by discipline. In any case, think of a primary source as first-hand knowledge, eyewitness accounts, reports or testimony about (X topic).
Here's a great definition from the American Library Association:
"Primary sources are original records created at the time historical events occurred or well after events in the form of memoirs and oral histories. Primary sources may include letters, manuscripts, diaries, journals, newspapers, speeches, interviews, memoirs, documents produced by government agencies such as Congress or the Office of the President, photographs, audio recordings, moving pictures or video recordings, research data, and objects or artifacts such as works of art or ancient roads, buildings, tools, and weapons. These sources serve as the raw material to interpret the past, and when they are used along with previous interpretations by historians, they provide the resources necessary for historical research."
--Using Primary Sources on the Web, rev. 2008.
A secondary source is based on other sources. It includes analysis, criticism, or other intellectual input. Review articles are based on analysis of the published 'literature' (books, articles and dissertations about the topic). Secondary sources can include books, book chapters, articles, especially literature reviews, and some book reviews.
A tertiary source is commonly a resource or tool that helps people find primary or secondary sources. Tertiary sources include most bibliographies, databases and indexes, and library catalogs.