So, you want to know how opamps work or where they're used?
Either question is easily answerable by Google searches. Off the top of my head I can name a few (tens) of applications of opamps: - inverting amplifier - non-inverting amplifier - voltage adder - voltage substractor - comparator - voltage stabilizer/regulator - Schmidt trigger - integrator - differentiator - ADCs - DACs - oscillators - instrumentation amplifier and in many many many other more specific applications.
This is a book that popped up in Google searches about Analog Integrated Circuits (I would give you a link to the course I took in University on the subject but it wouldn't really help because it's in Romanian and it requires a username and password):
