Grinding and sanding refer to rather rough processing of a concrete surface with high abrasiveness. This step is usually followed by honing and, as the last step, polishing. Thereby, polishing implies using a very fine grit to achieve a smooth surface as a result (source).
However, the terms concrete grinder, sander and polisher are used almost interchangeably. The different manufacturers call the same kind of tool grinder as well as sander or polisher (some even combine these names) and they use these labels for hand-held as well as walk-behind tools.
When you intend to buy a sander, grinder or polisher, it is therefore important that you do not only search and select tools by the name but also pay attention to their specifications and characteristics. Thus, you will make sure that you are getting the type of concrete sander that you really need for the job.