Figure: I-V characteristics and output plot of a JFET n-channel transistor.
The FET manages the flow of electrons (or electron holes) from the source to drain by influencing the size and shape of a "conductive channel" created and affected by voltage (or lack of voltage) applied across the gate and source terminals (For easiness of discussion, this assumes body and source are related). This conductive channel is the "stream" by which electrons flow to drain from source.
A negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel, in an n-channel depletion-mode device. If the depletion region expands to fully close to channel, the resistance of the channel from source to drain turns large, and the FET is efficiently turned off like a switch. Similarly a positive gate-to-source voltage raises the channel size and permits electrons to flow easily.
On the other hand, in an n-channel enhancement-mode device, a positive gate-to-source voltage is essential to create a conductive channel, because one does not exist naturally within the transistor. The positive voltage that is attracts free-floating electrons within the body towards the gate, creating a conductive channel. But first, sufficient electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this makes a region free of mobile carriers called a depletion region, and the phenomenon is considered to as the threshold voltage of the FET. Further gate-to-source voltage rises will attract even much more electrons towards the gate that are able to create a conductive channel from source to drain; this process is termed as inversion.