If I could define street photography, I would define it as the act of taking photos of people in public urban areas in a candid manner. This means that when it comes to street photography, you don’t ask your subjects to pose for you. Instead of asking people to take their photo, you simply go up to them and take their photo. In addition, street photography integrates the people and their environment. To use an analogy, I think of street photography as a play, with the actors interacting with their stage in an interesting way.
Street photography also relies heavily on capturing “the decisive moment“. This means that the timing of the images is what typically makes it special–whether you got catch a person having a certain expression on their face or having a person doing something uncanny at a certain moment.
When I think of street portraiture, I would say that it is included under the general category of street photography–but it is its own sub-sect. Therefore the two are not mutually exclusive. Furthermore, I would further categorize street portraiture in two sections as well: posed vs candid. Although capturing posed street portraiture can capture strong images (think of The Satorialist), the truly exceptional images are those which are captured candidly (think Thomas Leuthard).
Now what makes capturing candid street portraits inherently better than capturing posed street portraits? Well to start off, it takes guts to take photos of people in public (without asking their permission). Frankly speaking, I think that street photography is 80% having the balls to go out in public and shoot strangers while only 20% is skill.