At the right you can see a schematic of how a JFET is constructed. There are two types of JFET 1. N-channel JFET and 2. P-channel JFET. Let's see how N-channel JFET is constructed. Take a N-type silicon, cut two slots at its left and right and pop in P-type semiconductor in those slots. What results is a N-channel JFET. Ohmic contacts are attached to the top and bottom and they serve as Drain and Source terminals. Ohmic contacts are attached to the P-type material too and they are both tied together and this is called as the gate.
P-Channel JFET has a similar construction where N and P regions of N-Channel JFET are swapped.