Raga - an aesthetic projection

The Raga is, as the wise declare, the sequence of musical notes and the play of sound which delights the hearts of men.

"A Raga is an aesthetic projection of the artist's inner spirit; it is a representation of his most profound sentiments and sensibilities, set forth through tones and melodies. But the notes of a Raga, by themselves have no vitality or force. The musician must breathe life into each Raga as he unfolds and expands it. A characteristic of the Raga, impossible to describe but brought to it by the performing artist, is the prana_the life. Through guidance of the Guru, and by his own talent and genius, the musician learns how to make the bare notes vibrate, pulsate, come alive."
(from 'My Music, My Life' - Pandit Ravi Shankar)

Raga - based on a scale

As for a slightly practical aspect, a Raga must belong to a scale, and, only those notes found in that particular scale pattern can be used in the given Raga. There are ten parental scales. Originally there were seventy two parental scales, a foundation that has not seen any change as far as the South Indian Classical music in concerned. In the North Indian Classical music, these scales are called Thaat. One can imagine the number of combinations and compositions that can come out of these seventy two scales. Out of the myriad possibilities, only several hundred Ragas are in use today. It is evident why the artist's search does not end in one life, and the thirst is carried forward from life to life, from generation to generation.