Web Center for Social Research Methods
Research Methods Knowledge Base: Sampling in Research
Site discusses the use of sampling in research. The key statistical terms used in sampling are linked for the user to quickly find definitions. The major concepts of probability and nonprobability are also highlighted.
Web Center for Social Research Methods
Research Methods Knowledge Base: Probability Sampling
This site from Cornell University contains great information on random or probability sampling. Definitely a site worth checking out on the subject.