[abruzzi]

12 tone is a historical scale. Initially (Pythagoras) the scale was based on perfect fifths. Over time the notes in between the diatonic scale note were added. Tuning started becoming a big deal because if you tune all your fifths to be perfect, some intervals outside of the scale got really weird. Equal temperament was the compromise solution.

Instead of tuning perfect fifths, you just make every half step the same. That means that your other intervals are all out of tune, but it is small enough that you don't have some keys that are horrible. Bach's 24 preludes and fugues (in each major and minor key) was partly a demonstration of a similar tuning system--well tempered, also known as Werkmeister.

Other music traditions such as indian classical/raga, use different tunings.

I'll also add that the all-key tunings like equal and well tempered are hugely important for the development of music from back to today, its not mere esoterica. Look at something like a late Beethoven piano sonata. In the course of a single piece, it may modulate a hundred times through a half dozen keys. The development of all key tunings, while they made 5ths worse, made it possible for a pieces of music on a fixed tuning instrument like a piano to range through any key, and that was huge to the development of music.