Well, the requirements are a bit complicated, but it generally breaks down as follows:
You must be a lawful permanent resident (i.e. have your "green card").
Must have resided in the U.S. for no less than 5 years.
Must not have had an absence from the U.S. for more than one year.
Must have actually been physically in the U.S. for at least 30 of the 60 months of your residency.
Reside in a state for at least 3 months.
You must also have a basic proficiency in English, have a good understanding of U.S. history, and be attached to the principles found in the U.S. Constitution. You must also be a person of good moral character, which means if you're guilty of certain crimes you'll most likely be denied citizenship.
The time requirements can be lessened for those married to U.S. citizens.
Pax,
Chris