The stock market is a platform on which shares are bought and sold and traded. Stocks are rated / valued by the use of indices like NSE and BSE. SEBI is the governing body that ensures safe and fair practices are followed and no rules are broken. A company offers its shares to the public for the first time by a process called IPO. Shares may be bought but cannot be retured at any point. Shares may be transferred. People / institutions may choose to invest or simply trade in the stock market.