What are they?
“A trademark can be any word, phrase, symbol, design, or a combination of these things that identifies your goods or services. It’s how customers recognize you in the marketplace and distinguish you from your competitors.” –US Patent Office
In it’s most common form, it’s a logo.
How do I get one?
As soon as you create the icon and start using it with your goods or services, you’ve become owner of the trademark.
If you want additional legal protection, you should register your trademark with the US Patent Office.
TM or R?
Anyone can claim TM on their symbol. Only once you’ve registered your Trademark can you use the R.
Your trademark typically only offers protection against similar marks in your field or industry. It typically does not provide protection against a similar mark in another industry. Consult with your attorney to strategize on protection.