What are Expression Trees in LINQ?

What are Expression Trees in LINQ? Could someone point me to a resource that has code and explanation?

From stackoverflow

• http://www.interact-sw.co.uk/iangblog/2005/09/30/expressiontrees

  http://msdn.microsoft.com/en-us/library/bb397951.aspx

• One more for you: C# and LINQ.