Difference Between Arduino Uno and ATMega 328P

..."In short an ATmega328 is a microcontroller chip found on Arduino Uno boards. ATmega328 microcontrollers are from the 8-bit AVR microcontroller family. The image below shows an AVR ATmega328 microcontroller chip. The exact part number of this chip is ATMEGA328P-PU as found printed on the top of the chip.

The ATmega328 plugs into a socket in the Arduino Uno as shown in the image below. There are some Arduino Uno boards that have a surface mount ATmega328 chip. In this case it is a small square chip soldered on top of the Arduino.

On an Arduino, the microcontroller is the main component that does all of the work. When an Arduino sketch is loaded to the Arduino it is actually loaded to memory inside the microcontroller chip. The microcontroller then runs or executes the sketch when the Arduino is powered up or reset after programming."...

..."An ATmega2560 microcontroller is the microcontroller found on Arduino MEGA 2560 boards. The ATmega2560 microcontroller is also from the 8-bit AVR microcontroller family. It differs from the ATmega328 in that it is only available in a surface mount package, so can't be inserted into and removed from a socket on the Arduino. It also has more memory, more pins and more built-in hardware peripherals than the ATmega328.

The image below shows an Arduino MEGA 2560 with ATmega2560 microcontroller chip."...

So the microcontroller is the memory chip for the Uno...