For visitors to Seoul who are planning to use its public transportation, take note that the authorities have revised the bus and subway systems effective 1 July.
In a nutshell, they've colour-coded the buses to indicate different service routes, changed the numbering system, created special buses-only lanes on major streets, introduced the T-Money smartcards for cashless payments and the cruncher, increased fares(!!).
FAQ on the new transportation system
Here are some related links:
Summary of the revised bus system (pdf file, from Korea Herald)
A guide to the revised bus system (pdf file, from Seoul Metropolitan Government)
Seoul subway website