A demat account allows investors to hold securities like stocks in electronic form instead of physical certificates. It involves converting physical certificates into an electronic format to make transactions faster and safer. Key benefits include immediate transfer of securities, reduced paperwork, and elimination of risks like loss or theft of physical certificates. Major depositories that provide demat services in India are National Securities Depository Limited and Central Depository Services Limited. Maintaining a demat account is necessary for buying and selling shares through stock exchanges according to SEBI guidelines.