How is a quantum computer different to a normal computer?

Question: How is a quantum computer different to a normal computer?
- Keywords:
  computer,
  quantum,
  subatomic,
  technology
Asked by imlaurenpontin to Chris on 17 Jun 2013. This question was also asked by harryphillips4, meesh.
- Chris Mansell answered on 17 Jun 2013:
  
  Normal computers behave according to the laws of every-day-physics. Quantum computers are made out of things that exhibit highly quantum-mechanical behaviour. This is the defining difference between normal computers and quantum computers. The consequence of this difference is that quantum computers can do lots of calculations very efficiently.
  
  In some of the other questions I have answered, I give more details about what sorts of calculations quantum computers are good at tackling.

Question: How is a quantum computer different to a normal computer?