What is street photography?
According to Wikipedia it is "Street photography is an art photography that features the human condition within public places and does not necessitate the presence of a street or even the urban environment. The subject of the photograph might be absent of people and can be object or environment where the image projects a decidedly human character in facsimile or aesthetic."
http://en.wikipedia.org/wiki/Street_photography
Well there you have one opinion, I personally think it is any photo taken in a public space that depicts the human condition in all it's splendour.
Like Robert Capa said " The pictures are there and you just take them."
Street photography is documenting the present in photographic form and preserving it for the future, it's an art form much like many others with the simple difference that the artist has very little control over his work, the streets predict the outcome of the subject and all you have control over is the composition and timing.

Photo by: Jan TruterThe streets are ever changing and there are no do-overs and that is what makes it all so exiting ,take the above image for example these two were not there a moment ago and a few seconds later they were gone, this image will never happen again and if it wasn't for the photographer being there at that exact moment (timing) it would have been lost for all eternity never to be seen like this again.(Documenting)
What street photography definitely isn't is professionally posed or studio photography, street photography is not sterile and staged it has to have a certain amount of candidness to it but in saying that I don't mean you cannot ask someone to take their picture sometimes that is unavoidable and there is a difference between a street portrait and a studio portrait so if the situation calls for it feel free to interact with your subject before taking the shot, maybe he/she has an interesting feature or piece of jewelry and you really want to capture it but without their involvement it would be really difficult to capture so go up and ask if they would mind you taking the photo,do your thing smile and say thank you, maybe give them your business card and leave.Simple as that
Many of the masters of street photography interact with their subjects and their work is still considered to be "street" so don't be an ass and always stalk your prey it is totally cool to interact and ask permission.
There are literally hundreds of different opinions out there about just what street photography is and a lot of them have valid points but not everyone agrees about everything, the above is my view on it and is by no means the only one but let us all just agree to disagree and continue making cool photos of the streets and the people and things in them.