An offer letter is a document which is given to a candidate after he has been selected for the position. The letter clearly, mentions the salary package, designation, department and other benefits that he will be entitled to, if he joins the company. Other than this, a statement of at-will employment, list of contingencies, and a confidentiality agreement.
A signed offer letter doesn’t mean that you are legally bound to join the company after that. However, that may be possible in very rare circumstances.