Raga is the basis for Indian classical music. There is no parallel to raga in any other system of music in the world. Raga is not just a scale or mode. Each raga is unique, has a specific pattern of ascending (aroh) and descending (avroh) notes, has a dominant (vadi) and a subdominant (samvadi) note of its own. Each raga has its own form, color, mood and a sentiment to express. Ragas have particular times and seasons assigned to them. Thus, there are ragas for every hour of the day and night and also for every season, festival and occasion. By virtue of permutations and combinations of the seven principal swaras (tones) and five upaswaras (semi-tones), there could theoretically be 34848 ragas. If we take into account ragas with zigzag ascending and descending orders, their number would be almost infinite. However, only a few hundred ragas are now in use.
Ragas are classified under ten parent scales (thaats) in the north Indian or Hindustani music system, and under seventy two parent scales (melakartas) in the south Indian or Carnatic music system. The ten thaats in Hindustani music are Asavari, Bhairav, Bhairavi, Bilawal, Kafi, Kalyan, Khamaj, Marwa, Poorvi and Todi. Ragas in a particular thaat share more or less the same tonal structures, same rasa (sentiment) and the same prahar (3 hour time periods of day or night).
There is no exact synonym for the Sanskrit word rasa in any language. It could be variously described as sentiment, feeling or mood. Every raga expresses one or more subtle shades of a rasa depending on the mood of the artist at the moment. Even though the rasa is broadly suggested by the raga, a seasoned artist never feels constrained and in inspired moments, gives vent to imagination and creativity to paint a very individual tonal picture of the raga. However, such is the power of the raga that before long it again takes over and engulfs the artist in its grip and dictates expression. This is the essence of what is called manodharma (impromptu individual creative and stylistic expression) in Indian music.
The Indian classical system of music is perhaps the most sophisticated and complex of all the systems in the world. It is very difficult to understand its structure. However, the lay music lover can always enjoy the beauty of the ragas even without the knowledge of the intricacies of Indian music. There is no bar for passive enjoyment.