Not everyone will want one of these or even use it, but its there for you to use if you want to. An order book is where you give us a list of the things you'd like to buy - but not right now, only when the time is right.
As example, suppose you wanted to buy shares in a celeb but there's no way you're going to pay 2,000 CD each - thats waayy too much!
However, you'd be happy to splash out for 10 of their shares at say 1,500 CD, so you agree to buy 10 of their shares but at the same time you enter the figure of 1,500. You need to be on the full website - not a mobile or tablet to do this.
Our system will create an entry in the Order Book for you and it will then watch the value of that celeb's shares and the moment they drop or rise to the amount you entered (in this example 1,500) it will then automatically purchase the amount you said you wanted.
However, if you haven't got enough cash in your bank when that happens - the sale won't go ahead.
So the Order Book is a way of delaying an order until another time.