An employment contract is an agreement between an employer and an employee that explains the employment relationship.
Contract terms vary from person to person and employer to employer.
With an employment contract, you may have more rights than if you were an at-will employee. Having an employment contract does not automatically mean you are not an at-will employee. Some contracts may specifically state that an employee is at will. Many contracts are ambiguous. The more limitations the contract puts on your employer's behavior, the more likely it is that you are not an at-will employee.