A market order is an order to buy or sell a stock at the current market price. Unless you specify otherwise, your broker will enter your order as a market order.
The advantage of a market order is you are almost always guaranteed your order will be executed (as long as there are willing buyers and sellers). Depending on your firms commission structure, a market order may also be less expensive than a limit order.
The disadvantage is the price you pay when your order is executed may not always be the price you obtained from a real-time quote service or were quoted by your broker. This may be especially true in fast-moving markets where stock prices are more volatile. When you place an order "at the market," particularly for a large number of shares, there is a greater chance you will receive different prices for parts of the order.