Hi! When you say 'на метро', it could be translated as 'go by metro'. So you just mention a means of transport. For example, you can ask: 'How do we get there?' and your friend may say: 'Let's go by metro.' When you say 'в метро', it means 'at a metro station'. For example, you can call you friend and ask: 'Where are you?' and your friend may asnwer: 'I'm at a metro station.'

Unfortunately this doesn't ALWAYS work the same way with a bus:) Sometimes we say 'я еду на автобусе' meaning the same thing as 'я еду в автобусе' (which is possibly more correct) - may be becaus автобус shares more with a cart or a horse:)

As evrybody said:) "На метро" is used as an adverb, often in combination with 'добраться', 'поехать' etc. It means mode/way of transportation. It works the same way (in the sense of: how we feel the meaning ) as Russian words in instrumental: Плыть морем. Добраться по воздуху (this one is not insrumental). "Туда можно добраться наземным транспортом или на метро". "Идти лесом" "забивать гвозди молотком" - I cite these two to illustrate my idea of 'adverbial' function. Think that 'на' here is a _continuation_ of the verb. It introduces something with semantics of 'how'. When I say 'еду в метро' i feel it differently: now 'еду' and 'в метро' are independent parts to some extent. I can say "я читаю книгу в метро' to the same effect.