A sales commission is a sum of money paid to an employee upon completion of a task, usually selling a certain amount of goods or services. Employers sometimes use sales commissions as incentives to increase worker productivity. A commission may be paid in addition to a salary or instead of a salary. The Fair Labor Standards Act (FLSA) does not require the payment of commissions.
A fact sheet regarding commissions is available from the Wage and Hour Division's website.
Read the federal statute on this topic.