What is a Double Displacement Reaction?

[Bashyboy]

What exactly is a double displacement reaction? I searched my textbook for the term--yes, I did check the glossary first--and it proved futile. My teacher alludes to this term and even has one power point slide to it. It says, "When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves." And that is all he has pertaining to double displacement reactions. Now, I did come across precipitation and gas-evolution reactions, and they appear to be similar. Could someone give me a good definition of this sort of reaction?

Thank you

 

[Yanick]

http://lmgtfy.com/?q=double+displacement+reaction