I. Definitions of a primary source:

For historians a primary source is the raw data from which they will draw conclusions and postulate theories. Primary sources include: official documents, letters, oral testimony, newspapers and other periodical publications, statistics, photographs, ships logs, etc.

The Library of Congress in conjunction with its American Memory Project has a nice tutorial on primary sources which will explain this for students of all levels. See: http://memory.loc.gov/ammem/ndlpedu/lessons/psources/source.html

II. Problems with primary sources on the internet

1. Web pages on the Internet are easily altered and thus text can be easily manipulated.

2. Web pages can be created by anyone and sometimes in an attempt to defraud or defame.

For example:

on the internet:

Martin Luther King
http://www.martinlutherking.org/

VS

Martin Luther King Center
http://www.thekingcenter.org

and air brushing extrodionaire

The Commisar Vanishes
http://www.newseum.org/berlinwall/commissar_vanishes/

but it also happens in print:

Hitler Diaries http://www.syntac.net/hoax/kujau.php

Hamilton, Charles, The Hitler Diaries: Fakes that Fooled the World. Lexington, KY: The University Press of Kentucky, 1991.

3. The quality and authenticity of the original must always be questioned. Some printed editions of texts and documents may leave out parts of texts (for economical reasons) or perpetuate previous editions mistakes.

For example:

Two versions of the Gettysburg Address (Nicolay and Hay)
http://www.loc.gov/exhibits/gadd/gadrft.html

4. Copyright is an issue for both print and web resources. Some materials are in the public domain and present no problem in publishing. Some materials require releases. Some remain copyright protected and cannot be used. For a nice discussion of copyright see:
http://cidc.library.cornell.edu/copyright

5. As with any source, watch for intentional as well as unintentional bias. Ask what is excluded and why? Is copyright a factor in what can be included? Was a reliable copy not available. Was something left out because the organizer thought it was uninteresting, embarassing, or unimportant?

For example: Martha Ballard's Diary

A Midwife's Tale
http://www.pbs.org/wgbh/amex/mwt/index.html