Think of an integreated amp as a receiver without the radio (tuner) section in it. Thats all an integrated amp is. They call it integrated because the idea is that they integrated a pre-amp and a power amp into one chassis.
It kinda works like this;
for ex, The output from CD player is fed into a preamp where it gets amplified a little. At this stage, tone controls and balance are thrown in here too. The signal leaves the preamp and goes to the power where it REALLY gets amplified to drive your loudspeakers.
so to sum up;
seperates are pre-amp and power amp in 2 different chassis.
integrated amp is pre-amp and power amp built into one chassis
receiver is pre-amp. power-amp and tuner section all in one chassis.