The IRFZ44N is a N-channel MOSFET with a high drain current of 49A and low Rds value of 17.5 mΩ. It also has a low threshold voltage of 4V at which the MOSFET will start conducting. Hence it is commonly used with microcontrollers to drive with 5V.
The IRF840 is an N-Channel Power MOSFET which can switch loads upto 500V. The Mosfet could switch loads that consume upto 8A, it can turned on by providing a gate threshold voltage of 10V across the Gate and Source pin.
The IRF520 is a Power Mosfet with 9.2A collector current and 100V breakdown voltage. The mosfet has a low gate threshold voltage of 4V and hence commonly used with microcontrollers like Arduino for switching high current loads.