Four great web sites that explain many of the terms related to distance education.

American Society for Training & Development: E-learning Glossary
http://www.astd.org/lc/glossary.htm

University of Idaho: Glossary of Distance Education Terminology
http://uidaho.edu/eo/dist13.html

Penn State World Campus: Glossary of Distance Education
https://courses.worldcampus.psu.edu/public/faculty/DEGlossary.shtml

eLearners.com: Distance Learning Glossary
http://www.elearners.com/resources/glossary.asp