For an N channel FET, just think of it as a voltage controlled switch. A voltage around 3 to 5 volts between the gate and the source will drop the drain to source resistance to 10's of milliohms. The gate looks like a capacitor between gate and source to the outside world.
For a P channel it works the same way except drop the gate below the source voltage by 3 to 5 volts and the drain to source resistance drops. If you take the gate beyond about 20 volts relative to the source (for most FETs) it "punches through" and blows up the FET. Think of it as an insulation failure that "arcs" over.. and "poof" you let out magic smoke.
Always remember the "body diode" caused by the intrinsic structure of the FET. There is a high power diode, typically rated nearly the same current capability as the FET from drain to source. For N channel Cathode to Drain, Anode to Source, reverse for P channel. This make as FET a great DC switch, but for AC you have to put two back to back or Source to Source.
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for securing the board then setting us FREE! Tech Tips in the WiKi: http://endless-sphere.com/w "Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God..." all the best, Dave