A habit is a learned behavior that becomes reflexive over time. It requires almost no willpower which is a finite resource. A habit is a life long behaviour.

A goal is an objective or target that someone is trying to reach or achieve.

For example in the case of dieting. Most people when they reach their goal weight, they revert to their previous state. They struggled, they achieved their goal, they used their willpower to do it. If they had built a life long eating behaviour, they wouldn't gain the weight back.