A limit order tells your broker to place a trade if you can get it at one price or better. For instance if you place a $20 buy limit order it tells your broker to get you in the stock as long as you can buy it for $20 or less.
If you can?t get in for that price you will not get filled and your order will expire.
A market order on the other hand just tells your broker, ?get me in this trade? you will get filled at the best available price whether it be $20 or $25 or whatever the stock is trading at. It insures your order will get filled, but it does not guarantee what price you will get filled at.
