CgCookie - Introduction to Retopology

https://cgcookie.com/courses/introduction-to-retopology

As modern workflows have incorporated more sculpting, the previous techniques of box-modeling and poly-modeling have seen less use. In place of these older workflows is a new synergy between artistically-driven sculpting and technically-minded poly-modeling, through a process known as Retopology. Retopology is the act of recreating an existing surface with more optimal geometry. A common use-case is creating a clean, quad-based 3D mesh for animation, but it's also used for most any final object that needs textured, animated, or otherwise manipulated in a way that sculpted meshes are not conducive to. Rendering animations in blender would be next to impossible without optimizing your blender models first.