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 BSS138 is an SMD Package (SOT23) Logic Level N-Channel MOSFET with drain current of 170mA and Drain Source Voltage of 100V. The MOSFET also has low threshold voltage of 1.7V typically.
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.