It is the process of buying and selling shares from the stock market. This is also called a secondary market. It is a place where already initiated shares are traded.
Stock Trading in basically buying and selling of company’s shares through different market. It requires two parties (person or company), a buyer (who want to buy the shares) and a seller (who holds the stock/shares and want to sell it).
Stock Exchange is a market place where all stock trading activities (buying & selling) took place. Company must be listed on Stock Exchange for stock trading through that exchange.