From the Blender wiki: "Retopo (remake topology) is a tool for remaking the topology of a mesh. It's the opposite of the sculpting tools: instead of reshaping the model but leaving the topology the same, it reforms the topology, but maintains the same shape as the original model. You will not change the geometry of the original mesh in any way, you will be creating a new mesh that is projected upon an existing mesh."
In this tutorial we will be taking a look at the retopology tools within Blender. The primary advantage to retopologizing a mesh is it allows you to easily create low-res models from high-res, sculpted models. The newly created low-res model can then be used to generate normal maps or displacement maps from the high-res model. This is particulalry useful in game developement. Let's take a look!