Kant differentiates between hypothetical and categorical imperatives. Hypothetical imperatives have conditions: "In X situation, I would have to do Y to get Z," or "At work, I would have to sell lots of shoes to get a promotion." Categorical imperatives have no conditions; these apply to all people in all situations no matter what the circumstances are.
The general definition of the categorical imperative is, "Act only on that maxim which you can at the same time will to be a universal law." An example that is not necessarily related to morality is that I would make it a rule (maxim) that I would pay my debts immediately so as to incur as little interest as possible. But speaking more in terms of morality, the idea is that the categorical imperative is always theoretically and practically applicable. In other words, it should always be morally and logically sound, and if I choose to adopt it as a maxim, I do so with the intention that all other people can and should adopt it as well.
What makes it very difficult to say what a categorical is or can be, is that these criteria for a categorical imperative often lead to irresolvable paradoxes. For example, suppose one decides to adopt the categorical imperative that "one should never lie." This seems like a righteous and moral maxim. But consider this scenario: I am in a house with a few friends. One of my friends in the house is named Bob. I know that there is a killer on the loose who, insanely, goes around killing people named Bob for no reason. If the killer comes to my door and asks if there is a Bob in the house, I am obliged by my categorical imperative of never lying to say "Yes, there is a Bob in here." But clearly, the logical and moral thing to do is to lie to the killer and say there is not a Bob in the house because I don't want Bob to die.
This is the problem with the categorical imperative; it is difficult to establish a universal (applying to everything at any time) law because conditions change and sometimes make the law function for illogical and/or immoral ends which is the opposite of what a universal, moral law is supposed to be.