In the Seoul area, we (meaning 'most people') use a card called "T-Money" for transit; I use it for the subway system, but many people also use it for buses.
In subway stations, it is very easy and fast to load money onto the card using machines designated for that purpose, then you simply scan the card at the subway checkpoint and exit (or when you enter and exit a bus).
I have known for a long time that I can use the card in taxis, but either forget to do so or simply prefer to pay cash,
While I also knew that the card could be used in some convenience stores, I did not know, until recently, that it could be used with some vending machines. Wow! I can buy a .70 cent bottle of water with my transit card! How cool is that?