[MJN]

If you really want to understand the subject there are numerous good books out there that cover the fundamentals very well in an organised fashion and will be probably be much less confusing than the kind of disjointed info that you may get from many posts here. Corky Bell has two good books one on supercharging and one on tubocharging that are excellent for understanding the fundamentals of forced induction.

Bottom line is that an engine is an air pump, the more air in the more power out. So if you can force air in at twice atmospheric pressure (14.7 psi boost) you will double the power output. Not quite true due to pumping losses and flow restriction etc but you get the idea.

A lot of people consider forced induction cheating, but once you've tried it you'll understand how you can easily become addicted to boost.