So what is Berlin school? First, let me tell you what it's not;
It's not ambient, dark ambient, electronica, krautrock, new-age or trance.
It could be described as a combination of 1950's - 1960's sci-fi movie space music and minimal music. Wikipedia used to have a page on Berlin school but they removed it. You can find an archived copy here.
Minimal of course, isn't ambient or new-age either. Proper minimal has the energy of rock. Ambient and new-age do not!
There is a nice two part BBC 4 documentary called 'Tones, Drones and Arpeggios, The Magic of Minimal' on the subject;