Alliance for Investors Education offers a good starting point to learn about how to invest.  It covers financial planning, stocks, mutual funds, bonds, futures, where to get help, young investors, online/internet investing, scams and older investors.

Campbell Harvey's Hypertext Finance Glossary

Campbell Harvey's Hypertext Finance Glossary offers a short, concise definition for finance terms.

Investopedia

Investopedia is a gateway for learning about investing.  It includes articles and insights, a dictionary, tutorials, and current news items.

Investor’s Information

Investor’s Information offers practical information regarding investing.  It includes publications and sections on “investing principles,” and “investor rights.”

Motley Fool

The Motley Fool is the place to start research on investing basics.  It includes articles, investment headlines, discussion boards, quotes and data.  Feature sections include information on investing strategies, the "fool school," retirement, and personal finance.

Smart Money University

This site offers online lessons and article on investing 101, taking action of your portfolio, strategic investing, retirement/401(k), college planning, short-term investing, and debt management.