Op-amps are a little hard to explain, but essentially they take something small and make it big!
Op-amps are perfect when you need to take a signal and have it go much higher than the original, for example doing audio amplification. Wikipedia
has a great history of the op-amp and it's development.
LM386 is made specificly for low voltage applications. Different op-amps will have different requirements, always check out the data sheet
! The LM386 can take 4V-12V or 5V-18V depending on the wiring and pump that up 20 to 200 times.
You can use this chip to build any of the following:
AM-FM radio amplifiers
Portable tape player amplifiers
Small Servo Drivers