A limit order is a specialty brokerage order type that allows you to specify a price at which to buy/sell.
For instance, if you want to sell 100 shares of XYZ, but think it might go up a bit more than the current price of $20, you could enter a limit-sell order at $22. That way, the broker would not sell the shares until the price reaches $22 or more. Depending on the volatility of a stock, you might get exactly $22 or you might get a bit more, say $22.10, as being the first trade at or above your given price.
On the buy side, you wish to buy 100 shares of XYZ, but don't want to pay more than $15 for them. You would use a limit-buy order to tell the broker not to purchase them for anything more than $15. This way, that will be the most you will pay.
Of course, the risk for both of these transactions is that the stock doesn't trade at or above (sell), or at or below (buy) your limit price. In which case, you end up keeping or never buying the shares. In other words, don't try to get too cute with this. If the shares are trading at $22.01, don't set your limit at $22.13, hoping to eke out another 12 cents of profit. It's almost always not worth it.
Depending on the broker, limit orders can be set to expire over different time frames. In other words, they will remain active until you either cancel them or the time runs out. Typical ranges are one day, one month, or GTC, which stands for "good 'til canceled: and really means three or four months.
Another aspect of the limit order is the ability to specify all or none at most brokers. If you are selling a block of shares an all or none requires that the whole block be traded. Otherwise, only a few shares may trade at your specified price over several days. Although most brokers charge a single commissions for all trades on one day, additional commissions are charged for trades on subsequent days. To avoid this, some limit orders are day orders that expire at the end of the day; others are good til cancelled, but usually specified after 90 or 180 days.
For how your broker handles limit orders in specific details, please contact it.