The adjective decided means recognizable, marked, or distinct. If you have a decided preference for classical music, you'll probably get Bach for your birthday and not the Beatles.
Decided comes from the verb decide. If something has been decided, the question has been answered. So when we use decided as an adjective, we mean that there is no question about it. If you have decided political views, you won't be swayed by any debate. Decided can also mean distinct, as in a decided splash of yellow paint on your red car.