Everybody's a photographer these days. All it takes is a smartphone, and never in the history of the country have more people been snapping shots of themselves, others, and, occasionally, scenery. So how does it fall out legally when you take pictures of people without their permission? It largely depends on where they are when you push the photo button.
Public Areas Are Fair Game
Is it illegal to take a picture of someone without permission? Generally, you can take any photos you want of people when they are in a public location, like a park, a beach or a city square. It's perfectly legal since they have elected to place themselves in a public location and have no reasonable expectation of privacy. If you snap a hundred pictures of people at a political rally, a marathon or a rock concert in the park, all is well and good.
People in public places have no reasonable expectation of privacy, including where they go, who they go with and what they do while there. For example, if you photograph a couple kissing on the beach and publish the photo in the newspaper, they cannot complain. They have no claim against you even if one of the two happens to be married to someone else and the marriage ends because of the photo.
Likewise, if you take photos of someone shooting up drugs, wearing a bathing suit three sizes too small, falling down drunk or smacking their baby, you are within your rights. The exception is when the person being photographed is in some particular part of the public space where he has a reasonable expectation of privacy.
In some places, even though they are public, most people expect to have privacy. If these expectations are deemed reasonable by the courts, you cannot photograph other people in these spots. People have been found to have reasonable expectations of privacy in places like public bathrooms, changing lockers at the pool and trying-on-clothes cubicles.
And people almost always have a reasonable expectation of privacy when they are at home with their families. That means that you can't take pictures of people inside their own home without their permission. The rule applies even if you are standing on a public sidewalk looking through a window and using a telephoto lens. This kind of photo violates their rights.
Private Property Depends on Owner
The owner of private property, like a store or theater, controls whether you can take pictures there or not. Ask before you snap. The owner has the right to kick you out if you take pictures without permission and, if you refuse to leave, you can be arrested for trespass. This rule also applies to some businesses one thinks of as owned by the public, like a public library or a government building.