What is exactly the concept of the "whole tone scale"? Is it a scale of C - D - E - F♯ - G♯ - A♯ - C, made of only M2s and a d3?
It's a scale that consists (as per its name) only of whole tone steps (or Major seconds if you want)*, meaning that each note is a whole tone away both from its predecessor and its sucessor. More common scales have 7 notes each (heptatonic), but this one has only 6 (hexatonic).
Besides the common patterns that are usually used to create scales (major, minor and the modes), there are numerous other ways to create scales. The whole tone scale is just one of many examples.
The interesting fact about this scale is that it has no leading tone and all the triads (chords) are augmented, so the concept of tonality isn't as strong as it is in a major/minor scale.
Also, in the equal tempered tuning, there are only two whole tone, scales:
- C, D, E F#, G#, A#, C.
- C#, D#, F, G, A, B.
You can try it yourself, to see that on no matter which note you begin the scale, you won't get a different scale from these two (starting of course on the note you choose to begin).
Wikipedia has some examples of its use in compositions if you are interested.
*Think of A# as its enharmonic equivalent, which is Bb and is a step (Major second) away from the tonic C.