TM, SM or Ⓡ? What does a trademark symbol mean and when should you use it?

A trademark is a source identifier: it tells the public that these products and services come from your company.  The use of the appropriate trademark symbol - TM, SM or Ⓡ - next to your mark gives notice to consumers that your trademark is a trademark, that these aren't just words or designs.

It it advisable to use one of these symbols, just be sure you're using the right one and using it in the right way.

So, what do TM, SM and Ⓡ mean?