Other
Graph pad.com: Comparing Linear Regression to Nonlinear Regression
This resource presents a complete and detailed look at linear regression. This guide provides lots of details and graphs to explain each concept.
Wikimedia
Wikipedia: Linear Regression
Wikipedia explains the statistics term, "linear regression." Includes table of contents, hyperlinked terms, and formulas.
Wolfram Research
Wolfram Math World: Linear Regression Model
A basic definition for what linear regression is can be found here. Examples of forms of linear regression are also given.
Other
Stat Soft: Statistics Glossary
Dozens of statistical terms are defined and illustrated in this glossary.