Seoul Subway: click here for the link to enlarge the above image
Below is Busan's Subway Lines: click here to enlarge the image
Within each city, you will need to purchase a different transport card (or cell phone charm~which works in the same way as the card but hangs on your cellphone).
Daejeon: Hankumi Card & Cell Phone charm (below)
Seoul: T-Money Card
Busan: 1-Day travel passes for W3 000 (valid for the whole day~this works out as the cheapest option for
traveling if you plan on being there for longer than and hour)
All of the above travel cards can be recharded at every Subway Station (either by machines or tellers), or GS 25 (convenience store).