Plugins that make Reptopolgy easier
Here is a list of most of the better add ons. Notice decimate is the first one and is included with blender and simply reduces the amount of polys in the mesh. (Very useful)
SpeedReptopoly seems to be pretty good, used to be free, now 5$
He talks about why, and gives multiple examples. I like the rectangular pillars he uses to show why it's important to have enough polys in the right places. He talks about boolean and how when you use that it can cause n gons, or 5 point polys. He shows a simple way to fix that, but retopology is basically fixing this for clean animatable objects.
When to actually use Reptopolgy video. Things that are static or solid rather than animated and soft don't need this as much. So a rock as they describe doesn't need to be. Rather they use decimate to reduce the polys to keep poly counts lower.
A beginner level video, that shows you the process, and some of the tools like the bridge tool to connect polys.
This tutorial shows you how the blender modifier shrinkwrap can be used to get smooth topology.
At the end you can apply the modifier to the mesh if you need to.
I like Grant's tutorials they usually are quick and to the point. This uses blender with no reptopolgy plugins.
He uses the snap and merge poly, or vertex feature of blender. And also shows to make sure you click on the automerge icon.
He is using the shrinkwrap modifier to help the new mesh to snap to the underlying one.
This is using a technique that allows you to take the high poly info and make a normal map that is used on the low poly model. Great technique, and shows you how to bake a normal map too. (Uses a cage object).
This website goes over some of the add-ons and how to install them.