There are two types of “bumps:” voluntary and involuntary. Voluntary bumping happens when an airline has overbooked any given flight and a passenger voluntarily agrees to accept re-accommodation on a later flight. In situations like this, the gate agent will make an announcement to all passengers seeking volunteers to give up their seats for compensation. Airline flight vouchers are generally offered to entice volunteers.
If volunteers cannot be found, the airline will begin the process of involuntarily bumping passengers. These passengers will still receive re-accommodation on a later flight and cash compensation.