FETs are devices with 4 terminals:

• source
• gate
• drain
• body (aka bias/base/bulk/substrate)

FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source

• length of the transistor is the distance between the source and drain
• width is the extension of the transistor, in the direction perpendicular to the cross-section in the diagram

Typically the width is much larger than the length of the gate.

A gate length of 1 µm limits the upper frequency to about 5 GHz, 0.2 µm to about 30 GHz