Law and common sense

In my home country, Sweden, you can take pictures almost everywhere you like, as long as it’s a public spot. The same is a fact in the Anglo-Saxon countries (USA and Great Britain). Other countries have different rules and in for example France you can only take picture of strangers in public areas as open squares and such. Other countries stipulate that you need consent from people to take pictures of them.
But basically, as a street photographer, you’re not invading anyone’s private space if they are in a public area.

In most countries it’s the publishing that has some hindrance. Most street photographers I know, get around this by claiming that their pictures depict art (culture) in a wider meaning.
Commercial publications fall under another category of regulations in most countries. In Sweden you always need written consent (release form), before publishing someone’s face or body while marketing something.

Between law and common sense
I always recommend people to take pictures with a large amount of common sense, because it’s sometimes quite easy to aggravate someone, even if you don’t break any laws. I also recommend walking about in a good mood, as a smile most often will resolve most problems.

In short
Remember: Most of the time you can take pictures without asking before. Publishing is another thing.

Links: On American soil and other countries: http://www.bypeople.com/photographers-rights-and-release-forms/