Some reading

What is a computer?

Something that computes.
http://en.wikipedia.org/wiki/Computer#History_of_computing
http://en.wikipedia.org/wiki/Babbage#Design_of_computers
http://en.wikipedia.org/wiki/Ada_Lovelace#First_computer_program
http://en.wikipedia.org/wiki/Von_Neumann_architecture

long term memory: e.g. hard disk
short term memory: RAM

ALU, arithmetic / logic unit
CPU: central processing unit, = ALU + control unit