You have an explicit contract. Employment which defines job requirements and pay. You sign it with your offer letter. It is usually very light in nature. As an

You have an explicit contract. Employment which defines job requirements and pay. You sign it with your offer letter. It is usually very light in nature. As an exempt salaried employee you can simply check your email and (legally, backed by case law) call it a day of work. In an at-will state an employer can let you go for any reason. The contract is favorable to both parties at the same time