Conference papers are original contributions that were presented at a scientific meeting. In most cases these are not or minimally peer-reviewed. The reason that scientists present a paper at a conference is to inform their peers of their work, especially their new results and ideas and to get feedback.
A conference is somewhat like an open market or bazaar - don't believe everything the "seller" tells you!
However, in some fields (e.g. computer and information science, public administration) major conferences are highly-regarded, the papers are strictly peer-reviewed, and the published proceedings are good sources.
But since conference papers can have different levels of reliability you have to be careful.
The original submissions, with no quality control; typically distributed at the meeting itself and "published" by the conference organizers are maybe not so reliable.
Sometimes they are also difficult to obtain after the conference is over. Avoid using this as a source if at all possible.
Edited Proceedings by a publisher reviewed by a scientific editor are reliable. And sometimes the papers are presented in a special issue of a scientific journal.
These should be considered as journal papers. They have an editor and may be cited as a whole; in this sense they are like an edited book.
Examples of conference proceedings
- ISPRS 2012 : proceedings of the XXIIth ISPRS congress : Imagining a sustainable future, 25 August - 1 September 2012, Melbourne, Australia. Vol. XXXIX
- Proceedings of the FIG Working Week 2012, Rome, 6-10 May 2012 – Knowing to manage the territory, protect the environment, evaluate the cultural heritage. Rome: International Federation of Surveyors (FIG), 2012. ISBN 97887-90907-98-3