The order book is a real-time display of all open buy (Bid) and sell (Ask) orders in the market. The green side shows buy prices, and the red side shows sell prices. The larger the quantities at each price level, the deeper the market.
When you place a limit order, a yellow marker (triangle) appears on the order book showing exactly where your order sits relative to the rest of the market.
A limit order waits in the order book until the market price reaches the price you set. The moment there's a match (a seller willing to sell at your price, or a buyer willing to buy at it), the order executes automatically.
You can edit or cancel open orders at any time through the 'Open Orders' section on the trading screen. Change the price or quantity, or cancel the order entirely. Full control is in your hands.