The definition of a primary source depends upon the discipline and on how one is using the source.
A primary source is a direct, first-hand account of the topic being studied. In an English class, you probably look at primary sources all the time. Some examples might include:
Original Literary Works
Manuscripts
An author's diaries, or letters
An interview with the author
The author's memoir or autobiography.
A secondary source would be something that interprets, analyzes, or remarks upon a primary source. Examples include:
Scholarly journal articles
Newspaper and magazine articles
Essays
Theses and dissertations
Stories or films produced about an historical event
A tertiary source is further removed from a primary source. It leads the researcher to a secondary source, rather than to the primary source.
Encyclopedias and dictionaries
Textbooks
Biographies