It is an extended reimplementation of Scratch (a project of the Lifelong Kindergarten Group at the MIT Media Lab) that allows you to Build Your Own Blocks. It also features first class lists, first class procedures, and continuations. These added capabilities make it suitable for a serious introduction to computer science for high school or college students.
There is some help on http://snap.berkeley.edu (without the /run suffix)
Check out the Reference Manual, which is also a concise introduction to the IDE: http://snap.berkeley.edu/SnapManual.pdf
and some introductory tutorials on Youtube:
About "Build Your Own Block: http://www.youtube.com/watch?v=Bbl2fh3igQ4
And on how to add inputs to them: http://www.youtube.com/watch?v=hZC2tDVYWBo
Also, there are helpscreens in the context menu for each block