Octopus card is a contactless stored value smart card for electronic payments in Hong Kong. It covers most of population of Hong Kong. With Octopus card, you could use it to pay for transportation fee of MTR, bus, minibus, ferry etc. Most of the stores and restaurants in Hong Kong are accepted Octopus payment.
There is a special tourist version Octopus card requires no deposit which cost HKD$39 and you could keep it as a souvenir after your visit. There is many places you could buy from, please visit the official site for information: