"CHESS ENGINES"

I see people here refer to Chess Engines constantly.

  Can someone please advise what these are.  Are they just the numerous computer chess programs that you play against which we are all familiar with or are they something entirely different? I see people referring to players getting help from chess engines and others refering to checking their games against chess engines.  

Where do you find these chess engines to check your game against if that is what people are using them for?

http://en.wikipedia.org/wiki/Chess_engine 

A chess engine is just the part of a chess program that calculates the moves. The engine is usually a separate executable program that can be installed into the larger chess program. That way, you can install engines of your choice into chess programs of your choice.

Some chess engines are sold, and some are given away for free.

Here are a bunch-o-links:

http://www.chessmosaic.com/ListOfAllEngines1.html