Margin Account
A special kind of account with a stockbroker that lets you borrow against stock you own, akin to a home-equity loan without the credit check. You can even borrow against stock to buy more stock (called leverage). But run a tab against a stock that crashes and you get a "margin call" from your broker that requires you to pay back the loan or sell the stock collateral. Last winter many brokers threw some cold water on the mania for Internet stocks by refusing to extend loans against volatile issues such as Amazon (AMZN) and Yahoo (YHOO).

Market Maker
A stock wholesaler that helps match buyers and sellers, in the process profiting from the "spread" between what potential buyers bid and potential sellers ask. Wild swings in the price of Internet stocks--Yahoo down 60 points, eBay up 80--have caused market makers to abandon the most volatile stocks, leading to even wilder price swings.

Real-Time Quotes
Up-to-the-second prices of the most recent stock sales and purchases. A hot commodity as recently as last year, real-time quotes have become a standard, free feature on the Web. The sec (see below) still places some restrictions on what used to be considered a professional service, though real-time quotes also scroll across the screens of cable channels CNBC and CNNfn. The next frontier: "streaming quotes," which update without clicking on Reload, and nasdaq Level II quotes, which open a window on the electronic trading floor and show haggling over the bid and ask prices, measured in "teenies" (1/16 of a dollar).

The Sec
The Securities and Exchange Commission is the stock police for the U.S., entrusted with ensuring some measure of fairness in the rough-and-tumble markets. Public companies are required to file regular reports with the sec (available online) about everything from quarterly earnings to stocks sales by company executives. sec enforcement also covers the abuses of insider stock trading (for example, when a buyer or seller acts on confidential information, such as a pending takeover) and stock promotion (for which the Internet is especially well suited).

Shorting Stock
The sale of a stock you don't own. The bet is that the price will drop before you actually have to supply the stock. The risk is that the stock will take off like a rocket. The sec tracks "short interest" in stocks, but its meaning is not always clear. Heavy betting against a company sounds bad, but sometimes it can actually cause a rise in a stock price if short sellers are forced to close out their positions. Last year these "short squeezes" helped explain the breathtaking run-up in Amazon and other Internet stocks, as nonbelievers scrambled to unwind their short sales. Professional "shorts" make careers of sniffing out businesses that have gone bad.

Si
Silicon Investor, one of the biggest online sites for stock-related discussion and general investor camaraderie. A membership fee is required before a user is able to post, raising the general quality of discussion, in contrast to the free-for-all on Yahoo's even busier stock-discussion boards.

Stock Options
The right to buy or sell stock at a set price before a given date. This can be enormously profitable if you have the right to buy stock for less than the going price. Many employees get these rights as company bonuses, but investors can trade call and put options for most big stocks. Options have a reputation for being risky, but they can also be used as insurance (for example, the right to sell a stock for a set price protects you against a sharp decline of that price). Options are a kind of derivative, so named because they derive their value from another investment.

Technical Analysis
The search for patterns intrinsic to the buying and selling of stock, in contrast to "fundamental analysis," which focuses on the actual health of underlying companies. Technicians are sometimes called chartists because of their devotion to entrail-like graphs that track movements in the price and volume of trades. The technical landscape has its own geography, using "trailing averages" to identify "break-outs," when a stock price moves out of its usual "trading range." Technical analysis plays to the computer's strengths, requiring reams of otherwise meaningless numbers and the processing power to give them an aura of rigor.