The difference between a BJT, or bipolar junction transistor, and an FET, or field effect transistor, lies in its control and polarity; BJT is bipolar and FET is unipolar. A BJT is primarily controlled by current, while an FET is mainly controlled by voltage.
A BJT relies on two different charge carriers, hole and electron for its conduction level. An FET relies on one charge carrier, either hole or electron for its conduction level. An n-channel FET has electron as its charge carrier. Hole is the charge carrier for a p-channel FET. BJTs can be npn or pnp transistors.
Other differences between a BJT and an FET are in terms of temperature stability and size. Normally, an FET is smaller with a more stable temperature than a BJT.