For historians primary sources are documents produced at, or near, the time of the event being studied. They can take numerous forms:
Since the variety of documents is so great and the definition so broad, it is always advisable to check with your instructors to see if the document you have is in fact a primary source by their understanding.
These documents can appear in a variety of formats--most notably, in print, online, and on microform (fiche or microfilm). Don't confuse the documents with the format in which they are available. Marquette University Archives has a huge number of original manuscripts, including letters and hand-written reports; but most documents like these are found reproduced in microform or online.