Contract of employment
A contract of employment is an agreement, likely written, between an employer and employee or group of employees. The latter is referred to a labor law where unions represent employees and negotiate a contract that sets things like pay, benefits and work rules for groups, or classes, of employees. This is also known as collective bargaining.
It's becoming more common to see a contract, or at least some written confirmation, memorializing the relationship between an individual employee and employer. Not too long ago such contracts were for higher level executives with complex bonus structures and agreed upon severance payments and conditions, commonly referred to as golden parachutes (like having a pre-nup except for employment).
A written employment contract can also avoid any confusion about whether the employee is actually an employee, or an independent contractor. This can affect protections provided by state and local laws, and determine whether the employer will contribute to the employee's social security expense at tax time.
You can find all of these (with examples) at: Employment legal forms
Hope this helps!