I am confuse with Package on package and Flip chip. Are they classified as multi-chip modules (MCM)?
Flip chip is about a single die. On the circuitry side solder balls are placed and the thing is mounted upside down on the PCB, hence the name.
Package on package (\$PoP\$) goes one step further: you have a flip chip, with another BGA on top of that, where the lower die occupies the place between the top die's solder balls.
Like many \$PoP\$ illustrations, this one shows a stacked die at the top, but that's not a requirement to talk about \$PoP\$.
A typical \$PoP\$ application would be to have a microcontroller below and a Flash or RAM device on top, since they will have to be tightly connected anyway.