Remesh is a high-level tool to let you quickly increase or decrease the mesh as you work, while the Refine and Reduce tools more purpose-built sub-tools for specific mesh operations. For example, use the Remesh Tool when you want to move back and forth adding and subtracting to the resolution of you mesh as you work, but if you know you don't want to decrease your mesh resolution, use the Refine tool. Similarly, use the Reduce tool when the only thing you want to do is remove polygons from the mesh.
You can increase or decrease the mesh resolution as you work with the tools.
To increase or decrease resolution on a mesh