This is an additional payment made to an employee for achieving single specific sales goals. This differs from a bonus in that it is directly tied to a single sales event (e.g. a set amount paid for every unit sold, as opposed to a set amount paid for achieving a cumulative target).
Commissions are usually given to employees within sales field but may also occur in other positions; instead of the employee getting paid per hour, within this arrangement, the employee is paid a commission based on how many sales or clients won; it may not always be the case, in some cases employees are commissions as well as a fixed salary. For example, call centre agents for a phone network might be given a target to reach a secure a certain number of customers, if the target is met a commission is paid on top of their salary based on a percentage.