The following is taken from the University Library site at U. of California Santa Cruz:
Introduction
Whether conducting research in the social sciences, humanities (especially history), arts, or natural sciences, the ability to distinguish between primary and secondary source material is essential. Basically, this distinction illustrates the degree to which the author of a piece is removed from the actual event being described, informing the reader as to whether the author is reporting impressions first hand (or is first to record these immediately following an event), or conveying the experiences and opinions of others—that is, second hand.
Primary sources
These are contemporary accounts of an event, written by someone who experienced or witnessed the event in question. These original documents (i.e., they are not about another document or account) are often diaries, letters, memoirs, journals, speeches, manuscripts, interviews and other such unpublished works. They may also include published pieces such as newspaper or magazine articles (as long as they are written soon after the fact and not as historical accounts), photographs, audio or video recordings, research reports in the natural or social sciences, or original literary or theatrical works.
Secondary sources
The function of these is to interpret primary sources, and so can be described as at least one step removed from the event or phenomenon under review. Secondary source materials, then, interpret, assign value to, conjecture upon, and draw conclusions about the events reported in primary sources. These are usually in the form of published works such as journal articles or books, but may include radio or television documentaries, or conference proceedings.
Primary sources are usually considered to be original documents, many being unpublished items like diaries and letters, though they can also be newspaper and magazine articles written very close to the time of an event or during the time period being researched. Yearbooks are considered to be primary source materials, as well. The following is a link to Yale University's page on primary sources, explaining them very well.
Here is a link to a site which gives examples of common secondary sources:
In some cases your teacher may ask that you include tertiary sources, some of which may include textbooks, dictionaries, encyclopedias and handbooks.
A tertiary source presents summaries or condensed versions of materials, usually with references back to the primary and/or secondary sources. They can be a good place to look up facts or get a general overview of a subject, but they rarely contain original material. (From Virginia Tech Libraries site: http://www.lib.vt.edu/help/research/primary-secondary-tertiary.html)
Many of our databases contain secondary source material. History databases interpret from primary source material. Magazine and journal articles often do the same, though depending on the date of the article, it could be argued that it is a primary source. Newspapers are usually the best kinds of published primary sources. You may search the historical New York Times database for articles which are presented as photographic reproductions of the original newspaper, or the Gale Custom Newspapers database in which the articles are presented in html format and do not include photos and other graphics which may have appeared in the original.