Stock trading allows the holder of the share to sell his shares while it allows a buyer to buy the share of company he/she wants to invest. Following are the key requirements:
1) One should have a trading / demat account with a broker as brokers act as an interface between the exchange and buyer/sellers.
2) Trading takes place when buyer and seller agree on a price for the share
3) Once trade is concluded buyer gets the share and seller gets the money. This is guaranteed by the exchange and the exchange ensures no counter party default takes place by taking adequate security deposits, margin money, volatility margin, bank guarantee etc.