A trading account and demat account is opened with a broker for which he charges commission.
Share certificates are stored digitally in a DEMAT account. A depository offers this service.in India there are only two depositories(NSDL AND CDSL).both brokers and depositories are interlinked. the bank ensures funds are transferred to trading account. when an order is placed electronically ,the exchange ensures that the shares are debited or credited to the respective accounts.