A correction to what Dave said. Even if you take a photograph of a person in a public place, and you own the copyright in the photo by virtue of being the photographer, you cannot always "do what you want" with the photo. Even when the taking of a photo may be perfectly legal, the way you use the photo may not be (or, at least, may give rise to a lawsuit that challenges the legality of its use).
There are laws, for instance, against using a person's image in a commercial manner without written permission from the person whose image you are using. People have what are known as "publicity rights" and they are able to control under certain circumstances how their image and name (and often voice and other aspects of their persona) are presented to the public.
I can, for instance, take a picture of a street musician who is performing in a public place, but I cannot (generally) use the photo of this street musician on, say, tee shirts or coffee mugs or posters or on products I wish to sell or in advertisements, without written permission from the street musician to use his image in that way. Without permission, I would be infringing on his rights. Even "fine art" photos that are for sale can give rise to a suit by a person pictured in the photo.
But, everything in law depends on the specific facts of a situation and, with your question, it could depend on the specific photos and the specific people pictured in the photos and how these specific people are pictured (ie. are they the focus of the photo or incidental to the shot?).
The wisest course of action always, whenever you are photographing people with an intent to display or sell the photos, is to get permission from the people pictured when possible. This is because, while inanimate objects and animals can't sue, people can sue and often will.
A simple release form can help save you from a future lawsuit.