Wolfram Research
Wolfram Math World: Standard Deviation
This World of Mathematics site by Eric Weisstein explains the term "Standard deviation." Provides step-by-step help, examples, and charts.
Other
Stat Soft: Statistics Glossary
Dozens of statistical terms are defined and illustrated in this glossary.
Other
Fayetteville State University: Standard Error
Standard error is discussed in terms of the Central Limit Theorem. The site mentions "Rule 3 of the CLT" as a means to eliminate the error. Here that process is validated in reason and supported with an example.
Other
Fayetteville State University: Hypothesis Testing
A page from the university's statistics site which builds on the standard errors from the Central Limit Theorem previously explained. Here hypothesis testing is defined and validated.