How do I deform one object using another object as a guide? I am not too sure if I am making myself clear, but is it possible to have, say, 
object A with 20 vertices and object B with 15 vertices. 

Is there an automated way to deform object A to resemble object B? I'm horrible at judging shape/sculpting and was wondering if there was an easier way to shape things.
A shrinkwrap modifier can be used to deform one object to the surface of another.

Retopology is a more manual approach that uses automatic transform snapping of vertices to another surface to make a clean duplicate of an object.