Primary sources are those which were either created during the time period being studied or were created at a later date by a participant in the events being studied (memoirs, for example). They reflect the individual viewpoint of a participant or observer. Primary sources enable the researcher to get as close as possible to what actually happened during an historical event or time period.
In the sciences, primary sources are original materials that have not been filtered through interpretation or evaluation by a second party. Examples include conference papers, dissertations, interviews, laboratory notebooks, patents, a study reported in a journal article, a survey reported in a journal article, and technical reports.
A secondary source is a work that interprets or analyzes an historical event or phenomenon. It is generally at least one step removed from the event and is often based on primary sources.
In the sciences, a secondary source would be a book or an article that analyzes or comments on the results of original research.
When using primary sources from the web, make sure that you have found a trustworthy website.
Disciplines vary when distinguishing between primary and secondary sources. Definitions that pertain to the Humanities and Social Sciences are quite different from those that apply to the Sciences.
If you're not sure whether something is a primary source, check with your instructor and/or consult a librarian.