Octopus Cards can be purchased at the Airport or at any subway and train stations. The card costs HK$150 (about US$18), which included a HK$50 refundable deposit for the card and HK$100 stored value. The value will be automatically deducted by a processor each time you use it. The card use Radio-Frequency Scanner, so you don't have to take your card out of your wallet or purse.
When the stored value is used up, you can reload value to it at any subway or train stations or at any 7-11 convenience shops. When you leave Hong Kong, you simply return the card to the octopus counter at the airport to get back your deposit together with any value left on the card.
Visit Octopus Holdings Limited.