When I was in high school, I took a college-level psychology class in which I learned about the id, the ego, and the super-ego.
According to Sigmund Freud, the id is basically instinct. It contains basic drives: the need to eat, sleep, reproduce, defend ourselves, etc. The id is the screaming, hungry baby, not caring who it is disturbing by its crying, not caring about anything at all until it has something to eat. The id is irrational.
Even after childhood, some people still have problems keeping their id in check. I think that everyone lets their id dominate at some point at least daily. The id is that first reaction we have to something, before we have a chance to let the ego and super-ego think about and ruminate on it. Some people never really develop past the id stage. Those are the selfish people of the world, who think only in terms of what is best for them and what they can get instead of what they can give.
The id is the devil on your shoulder, and the super-ego is the angel on your other shoulder. Most of the time, we would do well not to give into the id, but it can serve a good purpose at times. For instance, if you’re being attacked, it’s good to let your id take control so that you may defend yourself.
On a side note, there’s a company called id Software, which is known for the landmark first-person shooter games Doom and Quake. Perfect name for such a company, as their games involve quick, id-controlled thinking and destruction of enemies.