But

1. To suggest a contrast that is unexpected in light of the first clause: "Joey lost a fortune in the stock market, but he still seems able to live quite comfortably."
2. To suggest in an affirmative sense what the first part of the sentence implied in a negative way (sometimes replaced by on the contrary): "The club never invested foolishly, but used the services of a sage investment counselor."
3. To connect two ideas with the meaning of "with the exception of" (and then the second word takes over as subject): "Everybody but Goldenbreath is trying out for the team."