Questions about Photography and the Law

I am a new Photo Hobbiest. I have been taking alot of urban pictures in center city Philadelphia. I was wondering what the law concerning pictures of people? I was confronted by someone who told me they did not want my to use the picture I had just snapped because they are in the frame. I explained it is just their back (and their car without license plate). So to narrow my question down:

When do I need to get a release or something in writing to use a picture?

I appreciate your patience with me.

In order to use the photo for comercial use, ie: selling prints, stock photos advertising etc you must have a release for each recognisable person in the frame.

For editorial use, ie: news articles or your personal use.no release is required.

If you cant see who it is, or there is nothing readily identifyable about the person, no release is required.

^^^

its best generally if someone asks you not to, don't

be aware that 30th street station has laws about photographing inside it as well

www.freedomtophotograph.com could be a good resource for you