Short selling – A offers to sell B some shares of company X at a set rate some time in the future. A makes money buy buying the shares at a lower price between when the deal was arranged and when the shares need to be sold to B.
Trouble starts when the shares don’t fall in price sufficiently for A to buy the shares at a low enough price to make a profit and complete the deal. When this happens, there may be benefit in imagineering information about company X which could reduce its value (share price).
Market makers have an obligation to keep the markets fluid by guaranteeing the availability of certain amounts of some shares.
At least that’s my understanding of it…..
The Naked Trader is a reasonable source of information to get you started or put you off for life!