Determinism is the belief that human choice is an illusion, and we are entirely controlled by outside factors. It hides behind a grain of truth, which is that our brains and minds conform to the Law of Identity. Since this is true, the results of our minds are consistent with their nature. Which means our ideas are predictable (assuming one had a complete understanding of how the mind worked, and all of the variables involved).
The determinists claim that because the Law of Identity applies to the mind, the mind doesn't really exist. That mental action is an illusion. That because, given a choice, a person will choose one thing over another, it is claimed he has no choice. When confronted with the fact that we make choices everyday, the determinist claims that it is merely an illusion.
Determinism is merely a claim that there is no such thing as consciousness. The nature of the denial is a claim that if consciousness has identity, then it isn't real. On its face, this statement is absurd. It claims that consciousness is impossible, while the determinist claim to be conscious of the fact that it is impossible. The determinist argument is based on the assumption that to really have a choice, one must not be restricted by the laws of identity. It is a rebellion against reality.
Determinism has an interesting effect, not unintentionally. If people do not have a choice, there is no such thing as morality. Which means, there is no such thing as right or wrong. Which means, there can be no moral judgments or punishments for doing wrong. Defense lawyers are some of the biggest proponents of determinism. The crimes are alleged to be products of television, video games, guns, or bad parenting. Anything can cause people to commit crimes, but themselves.
As an aside, the Free Will side of the Determinism vs. Free Will debate often accept the irrational claims of the determinists. They accept that a consciousness with identity must mean the consciousness is only an illusion. They try to remedy this by claiming that the consciousness does exist, but has no identity. Or that some supernatural power (that lacks identity) influences the mind. This should not be taken as a sign that the determinists are right. On the contrary, it just means that if their false premise is shared by others, they too will be wrong.