It is the relationship between two unwanted signals. If they are from different sources they are usually uncorrelated. That means that if you multiply them together there will be no DC term produced. If they are from the same basic source like a clocked system they will have parts in common and multiplying them together will produce a DC component.
hope this tutorial helps u.. just did a google search and found its good! there are also many other books which explains very good!
Code:
http://iul.cs.byu.edu/450/F96/node22.html
and, correlation as the term implies denotes the relation between any two entities. if they are signals, relation between the two signals, incase of a noise signal, they indicate the relation between two noise signals. in general two noise signals are taken to be uncorelated for calculation purposes, unless or otherwise they are absolutely necessary. You could go for more info from any signals and systems book....