Junction FETs work with the diode junction in reverse bias, that is, a more positive voltage is applied to the cathode instead of the anode, the cathode being the gate terminal.
When a gate voltage is applied, the junction depletion region widens by action of the reverse bias of the PN junction. With enough voltage applied, the depletion region widens enough as to completely divide the P material bar, effectively preventing current from flowing. When the gate voltage is lowered, the depletion region shrinks again and current can flow again.
Even in the absence of a control voltage at the gate, the transistor is able to conduct current through its P material body, and works like a semiconductor resistor. When a gate voltage is present, it effectively increases the resistance of the JFET's body, thus controlling the amount of current flowing through it.
Since the PN junction of the JFET is in reverse bias mode, very little current flows (only leakage current caused by heat), so it is useful in applications where loading of a previous stage can affect its behavior or there's a need to limit the amount of consumed current, as in low power applications.
