The main difference between these two philosophies is that Kant’s philosophy is deontological while utilitarianism is consequentialist.
Consequentialist theories hold that the normative value of an action (whether it is good or bad) can only be determined by looking at its consequences. Utilitarianism says that an action is good if it brings a net gain in happiness to a society, regardless of the motive of the actor. Clearly, this is based on the consequences of the action.
Deontological theories hold that the normative value of an action comes from the degree to which it adheres to a rule or rules of ethical conduct. Kant provides the categorical imperative as a rule for conduct. Actions that are in accord with the categorical imperative are good regardless of what their consequences are.