SEBI is the regulator of stock exchanges in India, established in 1992 to replace the Controller of Capital Issues. It has various roles including licensing brokers and dealers, preventing fraud, regulating mergers and acquisitions, making new rules, educating investors, and auditing stock market performance. A Demat account allows investors to hold securities electronically through a Depository Participant instead of physical certificates and offers benefits like lower brokerage costs and increased liquidity.