AFAIK, They are just regular buses that take you from the airport to wherever (usually have preset routes that stop at certain destinations). you just find your bus, and get on it. They just call it limousine buses, but its nothing limo like (maybe foot rests), but in general, they are just buses. I have never been to the Busan Airport, but if it is like ICN, you just go to the bus stop, wait for the bus, and get on it.