At America's Gates by 
A primary source is a document or record containing firsthand information or original data on a topic. Primary sources are created contemporaneously with the topic they describe. Examples include original manuscripts, articles reporting original research or thought, photographs, drawings, memos, financial records, posters, film footage, interviews, government documents, public records, artifacts, and newspaper clippings. Primary sources also include first-hand accounts that were documented later, such as letters, diaries, autobiographies, memoirs, eyewitness accounts, and oral histories.
The definition for a primary source can be slippery because it depends on the discipline or even the particular research question. For example, if you are writing a book about newspaper publishing at the time of the constitutional convention in 1787, then a version of the constitution published in a newspaper at that time would be a primary source. However, if your book is about the constitutional convention itself, then that exact same newspaper article would be a secondary source while maybe James Madison's notes would be a primary source. Whatever documents are closest to the subject of your inquiry are what is considered primary.
A secondary source is any work that is one step removed from the original source. Secondary sources are created after the fact. Secondary sources describe, summarize, analyze, evaluate, or are derived from or based on primary sources. Examples include reviews, critical analyses, second-person accounts, and biographical and historical studies.
In the same way that a definition for primary sources is slippery so too is the definition for secondary sources. It depends entirely on the discipline and topic being examined.
