A FET is a field effect transistor.
A varactor diode is sort of a capacitor and a diode. All diodes have junction capacitance. This type of diode takes advantage of that property so that the junction capacitance of the diode changes with voltage. In other words, it is a capacitor whose capacitance changes with variation in voltage. It can be used to produce FM among other things.
xtal stands for crystal, so a crystal microphone.
Kind of hard to give you a rundown of what they do, they can be used in so many different ways. It would take way to long to explain it. You should get a book and read up on electronics if you want to understand how they work. yeah, they are very regular components that should be available at any decent electronics store, or online.