A raag (also spelled raga) is the central organising principle of Hindustani and Carnatic classical music. It is not a scale, not a key, and not a tune. It is a complete grammar for melodic improvisation — defining which notes (swara) belong to it, which are emphasized (vadi and samvadi), how the ascent (aroha) and descent (avaroha) must be approached, which phrases (pakad) instantly identify it, and which mood (rasa), time of day, and season it belongs to.
There are several hundred raags in active practice. Each is built from a parent scale called a thaat (in the Hindustani system) or a melakarta (in the Carnatic system). Within a single thaat, multiple raags can be derived by emphasising different notes, omitting some, or following distinct phrase patterns.
The performance of a raag is improvisational — an artist takes the rules and elaborates them across alap (free, slow exposition), jor (rhythm-introducing), jhala (fast-rhythmic), and finally a composition (bandish) in a fixed taal. The same raag, played by two artists, will sound entirely different in detail while still being instantly recognisable as that raag to a trained listener.