18+ gives you a 30% discount on the total price of a normal adult oyster card fee. 18.
Depending on which zone you would like to use for underground trains, the fee may change for example only zone 1 or zone 1 and 2 etc.
If you get a weekly pass for underground you'll also be able to uses the busses with no extra add-ons but for weekly bus pass you have to pay to use the underground.
Hope this help feel free to ask if you have any other questions.