An important provision is that a health insurance company can't deny coverage to an applicant solely because that applicant has a pre-existing health problem.
Another provision, the so-called universal coverage provision, mandates that all citizens should have health insurance. Those that don't have insurance face a fine. This provision is controversial and is the main reason the law is being challenged by those opposed to it. The U.S. Supreme Court is currently considering the constitutionality of the law.
One other provision of note is that insurance companies can't cap your lifetime coverage.