*Catching a bus* describes the process of getting to a stop/station, waiting and boarding. *Taking a bus* describes the entire process, including the journey itself. Much of the time, the distinction isn't important. For example: > "How did you get to work today?" > "I caught a bus." *(The listener infers that having caught the bus, you stay on it)* > "I took the bus." *(You have described the journey)* However, it could be relevant: > "I read a book while I was taking the bus" *(yes: reading while the bus is moving)* > "I read a book while I was catching the bus" *(unlikely: reading while stepping onto the bus, paying the driver, etc.)* It is a similar meaning to *catch* as catching a fish, or catching a ball. You and the bus are apart, and then you bring yourself into contact with it through your own effort.