A computer fan is any fan inside, or attached to, a computer case used for active cooling. Fans are used to draw cooler air into the case from the outside, expel warm air from inside and move air across a heat sink to cool a particular component. Both axial and sometimes centrifugal (blower/squirrel-cage) fans are used in computers. Computer fans commonly come in standard sizes, and are powered and controlled using 3-pin or 4-pin fan connectors.

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Most Computer Fans Can Be Classified by Dimensions:

80x80 mm Fans used primarily to cool a Larger Case and Used in side of Power Supplies.
80x80 CASE FAN
60x60 mm Fans used on Video Cards and Some Cases.
60x60 FAN
40x40 mm Fans used on CPU and Some Video Cards Basically to cool individual Chips with Heat Sinks on assorted Applications.
40x40 FAN
Fans for CPU and GPU.
CPU FAN

PCI Fans or Slot Fans Inserted into Slots on Back of Cases.SLOT FAN
PCI Slot FAN

FAN POWER and RPM Indicators.
Fans can include a 4 Wire Connector which uses the 5v or 12v to Power the Fan.
Fans that have a 3 Wire Connector include a Yellow RPM Indicator.