Indian classical music developed as a way to preserve the Vedas. The Vedas are ancient Sanskrit texts which form the basis of Hinduism. Although Hinduism developed what we now know as Indian classical music it is widely used across the Indian subcontinent regardless of religion. There are two schools of Indian classical music: Karnatic (also spelled Carnatic) and Hindustani. Karnatic is practiced in south India, and Hindustani is practiced all along the north of the subcontinent. A big difference between the two traditions is that Hindustani is very strict on compositions of raag and Karnatic is more flexible.
A raag is a composition of swara ('notes') said to have risen from Shiva's voice as he orated to a congregation of Gods in the name of mankind. Swara each correspond to animal sounds in nature and include the infamous Sa, Re, Ga, Ma, Pa and the oft forgotten Dha, Ni. A raag also contains rules on how and which notes can be changed or bent in the recital of the raag. The notes can be both played and sung. Raags are identified as male Raga and female Ragini which can be combined to form son and daughter raags. This familial imagery shows which raags are related and perhaps how they were derived.
The personification of the ragas also confers a demi-god association. It gives each raag a personality and a power, while also emphasizing the high regard Indian classical music earns amongst its people. For example, the raag Deepak if sung properly has the power to start a fire, or Megh has the power to start monsoons. Despite the many stories of the mythological prowess of raags, they do have the ability to illicit specific emotions and moods in the audience, such as anger, joy, disgust, etc. For this reason raags have in the past been restricted to being sung at specific times of the day.
So how does this all relate to Sikhism?
The individual hymns, or shabads, of the Sri Guru Granth Sahib (SGGS) were composed to and organized by raag. At the end of the SGGS is an index, the Raagmala ('String of Melodies') of all the raags that can be used in the recital of the Gurbani (literally 'Word of the Guru'). Each raag is associated with a specific time of day and sets a tone appropriate for that hour.
Guru Nanak Dev Ji used music to reach out to the people he preached to. Now kirtan, shabads in raag, is an important part of Sikh temple services. If you go for prayers make sure to enjoy the kirtan that is recited before the hukamnama. And enjoy it. :)
Raag @ Sikhwiki
SGGS @ Gurbani Files
Raga in SGGS @ Sikhwiki
Dolls of India