Trams were once ubiquitous in Istanbul. They were by far the easiest way to get around. Unfortunately, in the 1960s the city government made the misguided decision to take the tram system out of service. Instead of easing traffic congestion, this only served to make things worse. In 1990, the first line of Istanbul's modern tram was put back into service.
There are two types of trams in Istanbul. The newer trams are part of the city's mass transit system and feature comfortable, modern cars. The T1 line runs from Zeytinburnu in Besiktaş to Kabataş, where it connects to the Metro. It is 14km long and serves 24 stations, including Eminönü and Sultanahmet. This line is quite convenient for visitors, as it serves most major tourist sites. The T4 line covers several inner suburbs on the European side and mostly serves commuters.
There are two historic, or heritage, tram lines. The cars date from Istanbul's classic tram system. The T5 runs from the Tünel to Taksim. The T3 serves the historic Kadiköy neighborhood on the Asian side. These are mostly used by tourists and are especially good to use after a long day of walking.
Note that all public buses, trams, trains, and sea buses accept the AKBIL. There is a refundable 6TL charge for this device, which is much more convenient than buying tokens. Rates are slightly discounted and it can be refilled at booths in all the major stations.