A description is a statement that represents something in words in order to re-create a mental picture that represents a person, place, event, or action so that the reader may picture that which is being described. In short, to describe is to represent in words. (The etymological origin is from French and literally means 'I copy off, transcribe'.)

An explanation is a statement that makes something comprehensible by describing the relevant structure or operation or circumstances. Further, to give a sufficiently detailed report about (a) the reason for something, about why something happened, about a causal chain of events; about (b) how something works, about how elements in a system interact; about (c) how to do something, about the steps which need to be accomplished in order to accomplish a certain goal. (From French as well and means 'to flatten, or spread out'.)

So, for example, saying that, 'that combustion engine is 4 feet high, 150 pounds, shiny gray, loud, and hot' would be descriptive.

Saying the following explanatory bit would be a good start at describing how it works: air is initially introduced into a combustion chamber. The air is then compressed. This high compression heats the air. At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of the compressed air vaporizes the fuel. The vapor is then ignited by the heat from the compressed air in the combustion chamber. As the vapor reaches ignition temperature this causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft.

As was said: all explanation includes description but not vice-versa. You describe the components of an explanation. You may be making a fallacy of composition (the fallacy of inferring from the fact that every part of a whole is some-thing that the whole also is that same-thing [or something like that]). Just because an explanation is made up of descriptions does not mean that an explanation is only a description. (Just try that with an average American girl; here babe, I brought you a loose pile of pure carbon. What? A diamond! It's the same thing, no?)

Also, check this response out http://news.ycombinator.com/item?id=1630769