Retopo

"Retopology is a common part of modeling workflows. Often times, a model is created with emphasis on form and detail, however, its topology, or edge flow is not ideal, or the mesh is very dense, and not efficient. Modelers can create a new lower resolution mesh that matches the form of the original mesh."

At http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Meshes/Editing/Retopo