Like others, I found the Catechism's description of torture to have its plusses and minuses.
The plus is that it's very clear that the reason torture is wrong is that it "is contrary to respect for the person and for human dignity." (CCC 2297) As others have pointed out, there's no getting around this basic evil of torture.
But the minus is that other parts of the description are less than clear.
For example, Alex asked in a comment about the difference between moral and physical violence. This is, admitedly, somewhat technical language - though as Red points out it's not too difficult to sort through.
Violence: Violence is an act that violates something. That is, it is an act of direct injustice.
In our culture, we've come to associate "violence" with physical conflict and harm, but this is not, strictly speaking, what the Church means by the word. Most acts of physical conflict are violent, though not all. For example, beating an egg is not violent (in this way of speaking) because there is nothing violated in the egg; likewise, a surgeon harms a person's body (from a certain point of view), but again without violating the patient.
Moral violence, then, violates a person in his or her moral center rather than in his or her body. That is, it violates the reason and the will, the aspects of a person which enable him or her to be a moral agent.
In this way, lying is often an act of moral violence, because it deprives a person of the information needed to make a proper decision. (I'm thinking, for example, of the episode of Star Trek TNG in which Captain Picard is tortured by the Cardassians: "How many lights are there?")
Or, moral violence can be an attack on the freedom of the will, by threats or by denying legitimate options. Threatening to harm a prisoner's family members is moral violence in this way.
Most often, it seems to me, the physical violence aims at moral violence: sleep deprivation, pain, starvation, simulation of drowning, etc., all break down a person's integrity of mind and strength of will.
At least, that's the goal that I've heard from proponents of torture: get the prisoner to talk.
But this is also why torture is ineffective as an interrogation method: because when a person's mind and will are violated, they're capable of saying (or doing) virtually anything to relieve the torment. Such statements or actions are utterly unreliable as intelligence.