The motor-type category on the data plate refers to the
type of ventilation the motor uses. This includes the open type, which
provides flow-through ventilation from the fan mounted on the end of
the rotor. In some motors that are rated for variable-speed duty (used
drives), the fan will be a separate motor that
is built into the end of the rotor. The fan motor will be connected directly
across the supply voltage, so it will maintain a constant speed to provide
continual cooling regardless of the motor speed.
Another type of motor, the enclosed motor, is not air cooled with a
fan. Instead it's manufactured to allow heat to dissipate quickly to and from the inside of the motor outward to the frame. In most cases
the frame has fins built into it on the outside to provide more area
for cooling air to reach.