Brian Monetti, I took way too many science classes in college!
FET's (field effect transistors) are a type of unipolar transistor. The most popular type is the MOSFET, or Metal Oxide Semiconductor Field Effect Transistor. Check out this course, which has free materials that can help you learn all about FET's and MOSFETs: https://www.noodle.com/online-courses/oc3F/mos-transistors
Operational Amplifiers (shortened to OpAmp) are a type of high gain voltage amplifier, meaning that the voltage potential of the output compared to ground is significantly higher than the potential of the inputs. You can learn more about them here: https://www.noodle.com/learn/details/144027/operational-amplifier-tutorial-super-microphone-circuit