Package on package and Flip chip what is the difference?
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.
The top die is not a flip chip; note the bonding wires. The bottom die is a flip chip, though the soldered balls aren't visible; the die should have been 1/10mm or so above its substrate. 

  

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.