Excuse my ignorance, but could someone explain to me exactly what a turbo is and what it does?

I found a Honda Civic VX for sale with a turbo kit. I'm looking for a VX for the gas mileage, but some added power would be nice. How does a turbo affect gas mileage? Does adding a turbo lower the fuel efficiency at all?

Thanks in advance for your help.

EDIT: Also, what is an intercooler and what does it do?

http://auto.howstuffworks.com/turbo.htm
thats should cover your question on a turbos... and yes they do reduce gas mileage..

http://auto.howstuffworks.com/turbo.htm
thats should cover your question on a turbos... and yes they do reduce gas mileage..
BANG!

good link tons of neato stuff

How much of an impact do they have on gas mileage?

depends....on how much boost you run... more air requires more fuel....simple automotive math

well if youre on the gas alot then youll have lower mileage. but thats one of the good features about a turbo, it allows you to keep decent mileage.