Online discount brokers: This label is generally given to the companies you see on the list here. While discount brokers are increasingly offering "extras" like research on stocks and funds, they primarily exist to help you place orders to buy investments at a very low cost. Many investors don't need the hand-holding of a full-service broker, and would prefer to save money by paying no commission for online stock trades. That way they ensure more of their money goes toward their investment portfolio, not paying for frills.
A broker is simply a licensed person through whom you can buy and sell these stocks. When you use an online platform, it essentially acts as an online broker. If someone wants to buy any product, he can contact a brokerage service. They would place an order and let him know the market price of the product. If the investor is interested, they would include a commission for themselves and process the order forward. If the user wishes to buy stock, the broker would forward his order to a stock exchange, which would approximately take 3 days to complete including the money exchange between the broker and the investor.