Junction field effect transistors (JFETs) come in two 'flavours', p-channel and n-channel.
In each case a bar, or channel, of one type of semiconductor material is located inside a bulk material of the other kind
i.e. p inside n, or n inside p. The explanation given below assumes we are talking about an n-channel device. You can
explain the behaviour of the p-channel device by swapping everything around i.e. think of opposite signs for the voltage
and current, and replace electrons with holes and vice-versa.
Characteristic curves can be used to describe the behaviour of the JFET in detail.
To understand depletion zones, look at the information on diodes.
A pair of metallic contacts are placed at each end of the channel. When we apply a voltage between these, a current can flow
along the channel from one contact to the other. The contact which launches charges along the channel is called the
source, the one that 'eats' them at the other end is called the drain.
In an n-channel device, the channel is made of n-type semiconductor, so the charges free to move along the channel are
negatively (hence n) charged - they are electrons. In a p-channel device the free charges which move from end-to-end are positively
(hence p) charged - they are holes. Remember that a hole is the absence of an electron. In each case the source
puts fresh charges into the channel while the drain removes them at the other end.