The Indian Classical Music is claimed to be the oldest and uniquely unbroken musical tradition of our cultural heritage. The first subsisting description of this musical tradition, The Natyashastra by Bharata is appraised to be at least 1800 years old! In this book there is a chapter which is particularly dedicated to the depiction of the instruments, the system of the tala, and the rasas and bhavas, which are the sensations and feelings or emotions aroused by the tunes and scales.
Even the Ancient Greeks were aware of the emotional impact of the different scales. The boys in Sparta were allowed to study only the masculine and dignified major/ionian scale, as the minor, the lydian (kalyan that) scale was considered as something too licentious for a man's man. In India quite a few millennia were spent on refining how to present these emotions: generations over generations the most prominent musicians of their times were striving to conceive and preserve the previously accumulated achievements, and only the ones at their most sublime could add a bit to the flow of that tradition.
The Indian Classical Music is a musical language. It has its own sounds, words, sentences or poems, and last but not least it has its own grammar. What you see on the stage is not improvisation, but live musical communication. In the western culture we used to have this kind of mutual musical language in the baroque era, or recently in jazz. Jazz is nearly a hundred years old and think of all the changes it has lived through! The same could be said about the Indian Classical Music, although its rudiments - e.g. the system of the sounds in which the musicians play to convey the clarity of the emotions - have not changed since Bharata described them in his work.
In India two independent but compatible musical systems have evolved throughout the times: one is the raga - the system of tunes, the other is the tala - the system of rhythms. These are two complex musical systems which actually empower the musician to express his/her personality and inner world trough the music with the greatest possible freedom but without the violation of the musical system. Both systems are so complex and peculiar that if, let's say, everybody in the world decided to play the tabla, nobody would need to follow someone else's style!