Everyone lies sometimes. There are many reasons to lie. Sometimes we do not want to hurt someone's feelings. Sometimes we lie for our own advantage. We lie in business and in our personal lives.
Most adults are aware of it when they lie.
Young children sometimes have problems telling fantasy from reality. He may not even be aware that it is a lie. Children with good imaginations often take longer to learn what a lie really is. It is important to work with children to teach them about fantasy and reality, instead of punishing them for these kinds of lies. A pathological liar is somewhere between an adult and a child with a good imagination.