What is the Kant's categorical imperative?

Kant's categorical imperative is a test he created to see if an act is morally wrong. It simply states that 'an act in a particular circumstance is right if and only if you would choose that everyone perform the same act in under the same circumstances'. For example consider 'it is permissible to lie to further your own interests'. If we compare this to the CI test we draw the conclusion that lying to further your own interests is morally impermissible. It forms the basis of his universality of morality, that all humans can reason- think, their way to the same moral truths. Furthermore the CI is where Kant derives the idea of duties from. These, broadly speaking, are things humans are morally obliged to do.