When you purchase aircraft you generally place a number of firm orders. These firm orders are allocated production positions and are noted in the contract as to when they will deliver. You typically pay a non refundable deposit and progress payments against the final delivery price, so that when you finally technically accept the aeroplane you pay the balance owing and they throw you the keys and off you go.
You can then take options, which are delivery positions, held against a refundable deposit, and you generally have a date by which you need to exercise the option, after which the deposit reverts back to you or you can put it against a purchase right that you convert to an option.
A purchase right is an agreement which gives you the right to purchase an aeroplane at an agreed price (subject to escalations) but doesnt typically have a delivery position attached to it.
You can convert your purchase rights one airframe at a time into firm positions or options as you wish.
The purchase right typically doesnt convey any committment by the manufacturer to a delivery date.
It may be that you decide that you want to exercise a purchase right and go to manufacturer x to convert it into a firm position. The manufacturer says sorry, sold my last one to the bloke before you, I can do you a good line in those aeroplanes in say 2015. So you take a risk in that you may not get the delivery timeframe that you want - but you dont have 1 or 2% of the purchase price sitting as a deposit - so you need to weigh up the pros and cons.
For example, if you have options for the 777 with contractual delivery months in 2009-2010, you can be pretty sure that you will get the airframes then - but if you have purchase rights and go to Mr Boeing today he wont sell you one till say 2015 - by which time you may have missed the boat as it were.
Hope that helps ....