let us suppose you buy a share of tata motors from your trading account through call to your broker or by using a terminal , Once bought and the transaction is completed the work of trading account is finished. This shares you bought will require a place to get stored, now this shares will get stored in your demat account . So whenever you want to sell your shares, you can easily sell those from your trading account and it gets debited from your demat account . To open a DEMAT ACCOUNT you need to communicate with depository participants (DP) . However , nowadays you do not need to do anything just contact or choose the right broker like Asthatrade and they will help you in opening of your trading plus demat account .