The T-money card is a rechargeable smart card that can be used to pay public transit fares in the Seoul-Gyeonggi areas as well in other cities around Korea (bus, underground/metro, taxi) as well as purchases in designated areas.
The card sells for 2500 KRW and can be purchased in nearly all convenience stores and subway ticket booths. Users of the T-money card receive a discount on bus and subway fares (100 KRW) and can transfer without additional charges.
T-money cards allow you to transfer from one bus line to another, or from a subway to a bus (or vice versa) at no extra charge, as long as you board the next leg within 30 minutes of getting off the bus/subway. Just tap your card on the sensors as you get on and off the subway or bus, and your total fare will be calculated as if you were on one long subway/bus ride.