Quote from achilles28:
There's a short blurb on wiki. A google search for "margin interest" would do.
Banks loan money to broker-dealers (used for customer leverage), for the same reason they extend personal lines of credit, underwrite mortgages, or loan business money = to collect interest. It's no different than any other form of loan a bank offers. The reason it's profitable is because retail traders need more credit to trade than they have on account. Like NuketheWhales said, it's profitable for a b/d'er to offer it, because then they can make the spread on 1000 shares instead of 250 shares = more money for them. Extending credit to a broker to make leverage available to customers ensures a large line of credit or whatever is held by the broker/dealer at any given time, which accumulates interest in the banks favor. The reason why it's a low risk loan is due to hypothocation. In the case of a mortgage, a bank loans money to a home buyer, on the condition the bank "hypothetically" owns the home and can liquidate the property if payments go into arrears or can't be made. Similarly, with financial securities or commodities, those securities or instruments purchased by the customer are used as collateral against the leverage (read: loan) that was used to purchase those securities. It gets confusing using a forex example, a bank is loaning money to buy money.... But in the case of equity securities, it's easy. Say you want to buy 10,000 shares of MSFT on leverage. You broker loans you 360,000$ to buy 10K shares of MSFT at 36$ per share. The collateral on the 360,000$ your broker loaned you, is the shares you bought. If your loss on the position meets or exceeds your account balance, the broker liquidates those shares on the open market to recover it's loan to you (360,000$), your account is zero, the broker collects the spread on the transaction + margin interest, and the bank charges margin interest/line of credit to the broker. This is all spelled out in your broker agreement nobody reads (I didn't). The banks and broker/dealers make money. Retailer traders are only afforded *the chance* to make money. Retailers participate for the same reason people buy lottery tickets or go to the casino = chance to profit.
But don't get bogged down in the details of this. Like RunningBear said, it's got nothing to do with actually learning how to trade profitability. There's a good book by Larry Harris called Market Microstructure for Practioners. Again, unless you plan on working for a brokerage to collect fees, study the charts.