A limit order allows you, the investor, to specify the price at which your shares can be bought or sold by choosing a 'limit' price. As a seller, this means that your shares will not be sold at any price below your limit, while as a buyer this means that you will not be buying shares at a price higher than your limit. Typically limit orders are best for situations where getting a certain price is more important than the certainty of execution.
Market orders are transactions meant to execute only at the current market price. A sell will match the highest buyer and a buy order will match the lowest seller. There are no limits on US market orders, meaning that if the market price of a stock moves significantly, the order will move with the market in order to fill. Because of this, market orders can only be placed during US market hours when market price data is available.
Typically market orders are best for situations where the certainty of execution is more important than getting a certain price.
Stop-Limit Sell Orders
A stop-limit is a type of order which combines features of a stop (i.e. the ability to execute your order at a certain price) with a standard limit order.
When a stop-limit sell order is submitted to market, it will remain inactive until the market price of the security drops below a specified 'trigger' price. If the trigger price is met, the order will then become a standard limit order, and fill at the limit price or better.
You can find more information on stop-limit orders by clicking this link.