To summarize: --
Utilitarianism is a philosophy which assesses the morality of an action based upon whether its consequences serve the principal of utility, the greatest good for the greatest number.
Utilitarianism is therefore a consequentialist philosophy.
John Stuart Mill was the preeminent Utilitarian philosopher.
Non-consequentialism assesses the morality of actions based upon the actions themselves, not upon their effects.
Kant's Categorical Imperative is a non-consequentialist philosophy which states that actions are moral or immoral in and of themselves. To determine the morality of an action, we must consider what would happen if everyone were to act in that way. If an action is immoral in this general case, it is also immoral in every specific case.
As a general principle, people's human dignity should be respected because grave consequences would result if the opposite were held as a general principle. Since violating human dignity is immoral in general it is also, according to Kant, immoral in specific situations where violating human dignity would serve important aims; that is, people must not be treated as means to ends.