John and I bought a couple rechargeable cards called "T-Money" at a GS-25 (a convenience store) near our hotel. There were different types of cards, all were rechargeable and could be used in subways, buses, and even taxis and other participating store and vendors. And I believe using a T-Money card for subways also gives you a 10% discount on basic fair. Pretty cool huh.
With different types of the T-Money cards there were different added bonus benefits, discounts, and such, and some also come with coupons for different shops. The type John and I got were pretty basic, and if I remembered correctly it was 3,000w each for the cards. The clerk at the GS-25 was very helpful and helped us load 10,000w each on our cards. We didn't have to recharge again until toward the end of the trip. It was a steal.
The subway ticket and recharge machines were always conveniently located near the entrances and exits:
And this was one of the lines we took a few times - Line 4, which goes to Myeong-Dong, Seoul Station, etc.:
Our first subway ride in Seoul... we were going to Nandaemun!
One thing I saw that was different was that some people were selling things on the trains... I didn't see anyone buy anything, but the vendors were on the train quite a bit.
We didn't get to ride the buses around Seoul this time, but our multiple subway rides were all fun!
Oh, and the Seoul Cyber Station website was extremely helpful when planning our trip around town.