There is not one definition which defines street photography. Depending on who you ask or where you find your information, you will come upon conflicting responses. Some street photographers will say that it is about capturing the emotion and expressions of people, while others may put a higher emphasis on the urban environment. However I believe that the most effective street photographs are the ones that synthesize both the human element as well as the urban environment. To capture a moment in which a person is interacting with the environment or in which the environment is interacting with the person is a true mark of a skilled street photographer.
But when it comes down to it, it is basically taking photos on the streets. So instead of chasing sunsets and exotic creatures, you look for ordinary places and ordinary people and creatively compose them in a clever way. Anybody can take a good picture of a sunset. Although there are many technical details which go into capturing a perfect sunset, anybody can simply point their camera and capture a sunset which is inspiring. But when it comes to street photography, you must constantly be looking for either contrasting elements in an environment which make a photograph interesting.
Simply put, the main focus of street photography is taking the everyday and the mundane and making it into something unique and beautiful.
Update: Check out my new mini-documentary video on street photography below!