Street improvisation is where you perform some sort of acting or drama in the street. It is improvised because there does not appear to be any script. Some street improv is quite carefully planned but appears to the public to be spontaneous.
Flash mobs are a phenomena where crowds of people are organised via the internet or mobile phones to get together in a particluar place at a particular time to perform a particular act. It is usually an unusual or pointless act such as to turn up at a bookshop asking for books that don't exist, or to turn up and sing or dance, only to go on their way a short while later.
Improv Everywhere takes great pleasure in performing in public to entertain and put a smile on people's faces. All of their work is done simply to make people smile.
This form of street art is not really improvised, but highly planned. however, to the public audience who experience it, it seems quite spontaneous and really amazing.
You can see vidoes of all of Improv Everywhere's work on the Improv Everywhere Mission Page. Good ones to look at are the Star Wars Subway Car, Where's Rob, Some other great examples of street improv have been done by the team at T-Mobile in the UK.
T-Mobile Video of Spontaneous Singing in the Airport Arrivals
Video of Improv Everywhere performing a Scene from Star Wars on the Subway