Register and Style

This two items are contained in how two people from different places speak.

Register: Formal or informal, would determine if you are talking formally to a superior, your boss, your teacher, etc. Will be informal if the person you are talking to is at your "same level", like your best friend, your brother, etc.

Style: every person has its own way of saying things, his own choices of words for each situation, etc. This is called style.