On the order panel, you can choose to place a market or limit order. A market order will execute immediately at the current market price. A limit order lets you set your own price, as well as set some advanced order execution options.
Select 'market' above the buy/sell buttons to place a market order. Choose "Buy" or "Sell" and enter the size of your order. You can set the size in any supported currency. Selecting "Buy" for example, and entering 100 as the amount, then setting the units to US dollars will buy one hundred dollars worth of the digital currency you have selected at the market price.
Select 'limit' above the buy/sell buttons to place a limit order. Enter the order size, and your price, then select the button to place your order. When you place an order, it will be shown in several views - including 'Open orders', the 'Order book', and the 'Depth chart'. A basic limit order may be partially filled and is subject to price improvement - you may get an even better price than you asked for.
GDAX offers advanced limit order types.
When selecting a limit order, expand the "advanced section" to reveal the following Time in Force policies:
Time in force policies provide guarantees about the lifetime of an order. These types of orders provide advanced options you may be familiar with when trading traditional assets.