What does it really mean to be smart?

Well a lot of people think I'm smart because I get A's for everything. But my friend keeps telling me that just because I get good grades doesn't mean I'm smart. She says that smart people don't say stupid things like I do. Is she right? I don't think I say stupid things... I just have a sense of humour. I'm also quite childish, like I'm easily amused and impressed and I like to do childish activities. Does this mean I am not smart?