Many of the concepts of Indian music cannot be easily explained in western terms, and so it is that the most significant feature of Indian classical music is often the most difficult to explain and understand.
What is a raga?
Raga is at the heart of Indian music, "Ranjayati iti Raga" is a Sanskrit saying meaning, "that which colors the mind is a raga". As a blank canvas can be colored with colors and forms, so the receptive mind can be colored and affected by the pleasing and soothing sound of a raga. The beauty of the raga leads the listener to a specific state of mind. In this way the raga creates a forceful effect on the listener, permeating the senses with it's special mood and character. Ragas are fixed for different times such as early morning, late morning, afternoon, evening and night. There are also ragas for the different seasons such as spring, monsoon etc. Each of these ragas were designed to fit the feelings of these specific times.
A raga is not a scale, but a precise and highly organized melodic structure which follows it's own pattern of ascending and descending order, such as five notes up and seven notes down, for example. A particular raga may have a specific curve or zigzag movement that must be adhered to both in composition and improvisation. It is possible to have several ragas that function using the exact same notes. One difference between the ragas in this case will be the emphasis on certain notes and deemphasis of others. Every raga has a couple of notes that outweigh all the others. Also, some ragas being very serious are more suitable for a long performance and are played at a slow tempo. Lighter ragas will only be played at a medium or fast tempo and for a shorter duration.
There are thousands of ragas in existence but much fewer are in actual popular usage. Many ragas are similar in North and South Indian music but their names may differ, likewise many raga names may be the same in the two traditions but the ragas may differ.
Most importantly a raga must have the breath of life called prana, which can only be given by a sensitive musical artist. The notes of the raga and all their corresponding rules, by themselves have no vitality or force to give the proper feeling that comes in performance. The artist must bring themselves fully into the space of the raga and lose him or herself, giving a life force, so that the raga can unfold in all it's wonder. Through the guidance of the guru, the musician learns how to make the bare notes come alive, pulsate, and sing with life.