What is AKBIL ? - Getting around in Istanbul

Akbil, the 'smart card', is an electronic travel pass that can be used on all public transport except dolmus and minibuses. You get a 10 per cent discount on fares. Akbils are available for a small refundable deposit (TRL6) from booths at all main bus, sea bus and metro stations. To use it, firmly press the circular metal stud into the socket on the orange machine located next to the driver on buses, or to the left of turnstiles at all metro, light rail, tram and ferry stations. Recharge at booths by using 10 TRL or 20 TRL banknotes.

Particularly useful for visitors is the mavi (blue) travel pass valid for a day, a week, 15 days or a month. For current rates visit http://www.iett.gov.tr

Two or more people can use the same Akbil. There is no need for a traveling party to purchase more than one Akbil. Just use the one Akbil for each person as they pass through the turnstile. However, you may not receive the reduced fares for transfers if you use an Akbli for more than one passenger.

Source: http://www.timeout.com/istanbul/features/308/travel-information-getting-around-istanbul