If you hooked computer cooling fans to computers in the past, you have probably come across the well know DC 12 volts type of fans. The type of computer fan that has two wires, a red wire, the positive and the black wire, the negative, which you hook to a 12 volt source and the fan blades, will spin, such as a molex connector, from a computer PSU unit. This type of fan are idea for computer cases, as the 12 volts provided to the fans are highly recommend for that type of application, as their jobs is rapidly bring air into the case and exhaust the hot air inside the case out, from the back and a constant 12 voltage is required for that, to keep the fans constantly spin fast to keep the temperature inside the case nice and cool.
However, a CPU processor 3 pin fan works a little different to that fan. The CPU fan in total has 3 wires, 2 just like a regular 12V DC fan, except the third wire is called the 'tachometer' wire, which is on the third pin on the connector, which is plugged directly into the motherboard. That wire is used by the motherboard to detect the RPM speed the fan is spinning. This is so you can view the speed of the fan in software, such as 'Speedfan', that allows you to monitor the temperature and fan status.