What is SEC

SEC stands for US Securities and Exchange Commission.
Website http://www.sec.gov/index.htm

SEC is the governing body that governs stocks (or as they're called securities or equities). As per their website,

The mission of the U.S. Securities and Exchange Commission is to protect investors, maintain fair, orderly, and efficient markets, and facilitate capital formation.

As more and more first-time investors turn to the markets to help secure their futures, pay for homes, and send children to college, our investor protection mission is more compelling than ever.

As our nation's securities exchanges mature into global for-profit competitors, there is even greater need for sound market regulation.

And the common interest of all Americans in a growing economy that produces jobs, improves our standard of living, and protects the value of our savings means that all of the SEC's actions must be taken with an eye toward promoting the capital formation that is necessary to sustain economic growth.

The world of investing is fascinating and complex, and it can be very fruitful. But unlike the banking world, where deposits are guaranteed by the federal government, stocks, bonds and other securities can lose value. There are no guarantees. That's why investing is not a spectator sport. By far the best way for investors to protect the money they put into the securities markets is to do research and ask questions.

The laws and rules that govern the securities industry

One of the best things I found on the SEC website are the following calculators:
Mutual Fund Calculators
Tax-free versus Taxable Yield Comparison Calculator
Cost Savings Calculator
Loan Calculator
Savings Calculator
Mutual Fund Breakpoint Search Tool
Social Security Retirement Planner
Ballpark Estimate Retirement Calculator
519 College Savings Plan Calculator

All the calculators are at SEC's "Calculators for Investors" website.