MOSFETs and BJTs are both types of transistor. A transistor is an electronic component with three terminals that can be used as an electronically controlled switch or as a voltage amplifier. MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. BJT stands for Bipolar Junction Transistor. Both MOSFETs and BJTs are widely used in electronics and computer engineering.
MOSFETs have higher input impedance than BJTs. The input impedance is a measure of the resistance of the input terminal of the transistor to electrical current. When designing voltage amplifiers it is desirable for the input resistance to be as high as possible. Therefore MOSFETs are more widely used in the input stage of voltage amplifiers.
MOSFETs can be made much smaller than BJTs. Many more MOSFETs can be placed in a smaller area than BJTs. For this reason MOSFETs form the bulk of the transistors used in microchips and computer processors. MOSFETs are also easier to manufacture than BJTs because they take fewer steps to make.
MOSFETs are less noisy than BJTs. In an electronics context noise refers to random interference in a signal. When a transistor is used to amplify a signal the internal processes of the transistor will introduce some of this random interference. BJTs generally introduce more noise into the signal than MOSFETs. This means MOSFETs are more suitable for signal processing applications or for voltage amplifiers.
BJTs suffer from a property known as "thermal runaway." Thermal runaway happens because the conductivity of a BJT increases with temperature. Because transistors tend to heat up in proportion to current flowing through them this means that the conductivity and temperature of BJTs can increase exponentially. This can damage the BJT and makes designing circuits for BJTs more difficult. MOSFETs do not suffer from thermal runaway.