Market Order (MKT)
A Market order is a buy or sell order that your broker will try to immediately execute at current market prices.
See U.S. Securities and Exchange Commission Market Order topic.
See Wikipedia topic for additional info.
Limit Order (LMT)
A Limit order is a buy or sell order that your broker will try to immediately execute at no more (for Buy order) or at no less (for Sell order) than a Limit Price.
See U.S. Securities and Exchange Commission Market Order topic.
See Wikipedia topic for additional info.
Additional Information
U.S. Securities and Exchange Commission notes on:
See also: Trading using Trade Grid.