We get this question quite a bit (mainly because we carry both kinds of tools). Plated diamonds generally have only one layer of diamonds electroplated onto a metal substrate with another softer metal (usually nickel). Once that one layer of diamond has worn down, that's it for that tool. You either have to get it stripped and replated or buy a whole new tool.

This is a very simplistic view of plated diamond tools as there are various forms of diamond plating that yield varying results. Brazing is a particularly good form of electroplating that yields very long lifespans and consistent cutting quality from and electroplated one layer diamond. There are also companies that electroplate various layers of diamond, but opinions vary as to how well that actually works.

Sintered diamond tools usually have a softer metal matrix on the business end of the particular tool that has diamond layered throughout the entire diamond tip of the tool. As the tool wears down, you can "dress" the softer metal which removes a layer of the metal along with the worn out diamond and exposes a brand new layer of fresh diamond. Sintered tools have a much longer lifespan and cut rate than comparable electroplated diamonds, but they do come with a few caveats. Since the metal matrix holding the diamond is a softer metal than the general electrplated diamond substrate, sintered tools can easily lose their shape if one is not careful and doesn't take care of the tool. They also require a much more stringent water use policy to maintain their shape and cutting ability. Depending on the form, many sintered tools may have to be dressed professionally at some point in order to maintain their character.

All in all, there are advantages and disadvantages to either form. Which tool to choose will depend greatly on what you are trying to accomplish and what you need your tool to accomplish for you.