What is the difference between "comprising" and "consisiting of" in a claim's preamble?

"Comprising" is understood to mean that the invention consists of the following combination of elements, by themselves or in combination with additional elements.  "Consisting of," on the other hand, is understood to mean "the following elements and no others."

A third transitional phrase (rarely used) is "consisting essentially of."  "Consisting essentially of" means the claim allows additional unrecited elements, but only if they do not change the basic characteristics of the combination.