What is the difference between a cash account and a margin account?
Cash accounts are used by customers who pay in full for the cost of the securities purchased. Margin accounts are used by customers who are authorized to borrow part of an investment's total purchase cost from their brokerage firm. This loan from the brokerage firm to the customer is secured by the value of the securities in the customer's account. Customers generally use margin to expand their purchasing power. However, customers who use margin also run the risk that if the value of the securities that secure the margin loan declines beyond a certain level, additional money or securities must be deposited to the account in order to make up the value. A brokerage firm may sell part or all of any securities held in the account, without prior notice to the customer, in order to make up the value and meet the margin limit requirements. These "margin calls" may occur suddenly and investors should take care to understand the financial impact that trading on margin can have on the value of their accounts.
What kinds of securities can I buy online?
You can buy almost any type of stock, bond, or mutual fund online.
What's the difference between a market order and limit order? Is one better than the other?
With a market order the customer instructs his or her brokerage firm to buy or sell a stock at whatever the price is when the trade is executed, presumably as soon as possible. If the price of the stock is moving quickly and there is a delay in the transmission of the order, then the price at which the customer purchases or sells the stock may be very different than what the customer expected when the order was placed. With a limit order, the customer specifies the price at which he or she is willing to buy or sell. Limit orders can help protect customers from rapid price changes when markets are moving fast. However, there is the risk that the limit order will not be executed. Also note that limit orders usually cost a bit more than market orders.