I agree that reading about them is the only way to learn, my friend. But, to break it down to simple terms, BJT's have relatively low impedance at the base, where FET's are very high impedance devices. BJT's are current controlled devices, where FET's are voltage controlled. Certain circuits can be designed with either type and made to work, but the properties of these devices LEND themselves to certain applications. FET's are particularly good in digital circuits, where voltage levels (i.e. 5v,0v) are used as signals. BJT's tend to lend themselves to analog applications. This does not mean, however, that a BJT can't be used in a digital circuit, and that FET's can't be used in analog, so don't be confused. Study more and learn more.