Health Care: What does the bill do?

One of the few things I know about the health care legislation is that it forces everyone in the country to buy insurance from an insurance company, with the idea that this will lower the premiums people pay because there will be more healthy people paying into the system, and this increased revenue (FOR THE INSURANCE COMPANIES) will subsidize the sick people.

My question is this: what's to stop the health care insurance companies from increasing the cost of the premiums for everyone? Does the legislation regulate the prices that health care insurance companies can charge, too? If so, how and by what standard?