İstanbul's public-transport system is excellent, and one of its major strengths is the İstanbulkart, a rechargeable travel card similar to London's Oyster Card, Hong Kong's Octopus Card and Paris' Navigo.
İstanbulkarts are simple to operate. As you enter a bus or pass through the turnstile at a ferry dock or metro station, swipe your card for entry and the fare will automatically be deducted from your balance. The cards offer a considerable discount on fares (₺2.60, as opposed to the usual ₺5, with additional transfers within a two-hour journey window: ₺1.85 for the first transfer, ₺1.40 for the second and ₺0.90 for all subsequent transfers). They can also be used to pay for fares for more than one traveller (one swipe per person per ride).
The cards can be purchased from machines at metro and funicular stations for a nonrefundable charge of ₺10, which includes ₺4 in credit. If you buy yours from a street kiosk near a tram or bus stop (look for an 'Akbil', 'Dolum Noktası' or 'İstanbulkart' sign), you will pay ₺8 for one with a plastic cover, or ₺7 without. These won't include any credit.
Cards can be recharged with amounts between ₺5 and ₺150 at kiosks or at machines at ferry docks, metro and bus stations.