https://randoof.wordpress.com/2011/09/05/the-arduino-uno-or-duemilanove-from-one-to-the-other/ , Diferences between Duemilanove and Uno.

The removable ATmega chip also means that when you complete a project and wish to use it, the chip can be removed and used in place alongside either a cheaper cloned version of the Arduino, or even make an Arduino yourself with the correct hardware, which is a lot cheaper than the Arduino and can be a lot smaller as well. For some idea of what can be achieved and for inspiration, have a look at this forum thread.

A microprocessor generally does not have Ram, ROM and IO pins. It usually uses its pins as a bus to interface to peripherals such as RAM, ROM, Serial ports, Digital and Analog IO. It is expandable at the board level due to this.

A microcontroller is 'all in one', the processor, ram, IO all on the one chip, as such you cannot (say) increase the amount of RAM available or the number of IO ports. The controlling bus is internal and not available to the board designer.