A market is an order type that allows you to purchase or sell stock at the best prevailing market price. Market orders are typically used when execution timing is valued over price.
A limit order allows you to specify the price you wish to purchase or sell stock at. Limit orders will be executed at the price specified or better; limit price or lower for buy orders and limit price or higher for sell orders.
Limit orders are typically used when price is more important than timing of execution as the prevailing price may not reach your limit price, which may leave your limit order unexecuted. Limit orders are valid until the next close of trading.