Some people are compulsive liars. What this means is that these individuals are addicted to lying. They just cannot stop lying. Lying is an obsession for them. Similarly, a person who is a compulsive gambler has to gamble, he cannot refrain from gambling. Even when he has run up a huge debt, he will beg, borrow or steal money and continue to gamble. He will be unable to kick the habit.

It was during the honeymoon that he realized that his wife was a compulsive eater.

Something that you do `impulsively', you do spontaneously. When a fast bowler bowls a bouncer, most batsmen impulsively duck. You may lie impulsively on an occasion, this doesn't mean that you lie all the time. In other words, you are not a `compulsive liar'. You are not constantly thinking about lying it just happens. A compulsive smoker or gambler constantly craves for the things that he is addicted to.