Contradict is to speak/act against what was earlier said/done. In usage, always, A contradicts B. So there always has to be a subject and an object. It can either be thoughts or actions: e.g of thought : Gavin decided one day that he will never trust Siva. But contradicting himself, he went ahead and believed Siva the very next day e.g. of Action : Gavin decided that he will never play football again. The very next day he was found playing football. e.g. While arguing against the topic in a debate, Gavin was soon found to be contradicting himself by speaking in favour of the topic.

When you make two statements and they conflict with each other. - I ate breakfast yesterday. - I didn't eat breakfast yesterday. Those two statements contradict each other since they can't both be true. They contradict each other.

As a verb it means to be contrary to, which means to go against something that was said or done. If you tell your sister you need to save all your money and then go shopping for three hours you have contradicted yourself. In most cases a contradiction is a negative thing to do. To understand it easiest, break the word down in two parts: contra=against and diction=choice of words used. I hope this helps.

Well, it's usually used when someone does the opposite of what they said they would do like, say a man gives a certain story to a law enforcement official, he then is asked again at a later date and he gives a different story that conflicts with the first one. That man has just contradicted himself.