Specifically, what happens if I put AC electricity in a DC brush-less CPU fan? I read some where that DC brush-less motors are like AC motors.
They don't, really. Perhaps you are thinking of the voltage. Generally, DC motors use a lower voltage than AC motors. This is due to the physical properties of electricity and how we transport electricity around a city. The energy used by a motor does not depend on whether the electricity is AC or DC. But the type of electricity, AC or DC, determines how the motor works.
You would blow it up. Brushless motors use solid- state commutation, and applying AC to electronics is a no- no.
