Did you ever wonder how 3D animators manipulate meshes with a gazillion polygons in them on computers mere mortals can afford? One method is called Subdivision Surfaces. You start with a comparatively low-polygon mesh called the “control mesh” and that’s the one you generally work with. The software subdivides and smooths that control mesh into the high-polygon monster that looks so good in renders–but only at render time.
Here’s an example: a cartoon bear mesh showing the polygon density at the subdivision levels between zero (where I work with him) and three (when he’s actually rendered).