This is a legal question and the best answer is to consult with a lawyer. Even in the US, there is the potential that some states laws are slight variants of each other.
The most known source of information on this is Bert P. Krages II who is an attorney and published a pamphlet about the photographers rights which you can obtain here and print yourself.
The gist is that with very few exceptions such as military installations you can photography anyone and anything you see in public where people have no expectations of privacy. Images taken this way belong to you.
What you can do with these photos is quite restricted because the image in the photo belongs to someone else. Because of this you cannot use such images for most commercial purposes, including licensing such images, selling prints of it or making advertisement from them. You can use these images for editorial use. See the above link for what constitutes editorial use.