Retopology is the process of taking a high resolution, high polygon digital sculpture and essentially recreating a lower polygon count model ontop of it. This lower polygon model is what will be brought into a real time game engine or can be much more easily animated compared to the very high polygon count mesh. The very fine details of the high poly sculpt will be what's known as "baked" onto the low poly that is unwrapped in the form of a normal map. Since a normal map is a texture map, baking is done during the texturing stage. Below is an example:
Zremesher Basics (good for organics)
Zremesher is a retopology tool inside of Zbrush that can be used to quickly generate a new mesh based all in quads. The tool has some limitations and is best used for smooth or organic objects.
Decimation Master in Zbrush
Decimation Master is a Zbrush plugin that is ideal for drastically lowering a digital sculpture's polygon count while retaining it's detail. The geometry becomes triangulated and is unsuitable for animation and baking, but it is a quick and efficient way of lowering the polygon count enough so that it can be imported into packages like 3ds Max to use as a proxy
Freeform Retopology tools inside of 3ds Max (good for hard surface)
You can of course use the standard modeling tools in 3ds Max to retopologize your high poly objects. The Freefrom modeling tools are specifically meant for something like this.