A market maker is a professioanl term used a for a professinal trader that works either through a connection on an exchange or on the actual floor of an exchange. Another way to think a market maker as is a broker because they that capablity but only to companies that want to be public and are already public.
Market makers get by what is called a spread. A spread is the difference between the Bid and Ask price people see everyday on all stocks. For example, the bid price of stock A is 1.00 and the ask price is 1.10, so the differece is 10 cents and that is the spread. The ask price is what the market maker is requesting for the stock and the bid price is what most investors want to pay, now that does not mean that the stock is worth the bid\ask. The bid\ask spread will change constantly during the trading day as news about the comapny filter through and the overall market, so always do research.
The difference of a stock is paid to the market marker. Remember that this may hold a lot of the stock and trades alot, so 10 cents on one share but on hunfreds to thousand shares a day will had up and the spread may be more or less on any given stock on any given day.