The differences among (R), TM, and SM are related to the registration status of the trademark.
(R)
You may only use the trademark registration symbol "(R)" if you have an Official Trademark Registration Certificate. You can obtain an Official Trademark Registration Certificate from the USPTO. The (R) symbol serves to inform others of your trademark registration. However, you cannot use the (R) symbol if you do not have a registered trademark.
TM
You can use the TM symbol for both unregistered and registered trademarks. The TM symbol conveys that you believe that you own rights in the trademark. Additionally, the TM symbol commonly symbolizes a trademark owners' common law trademark rights in the mark.
SM
The SM symbol stands for “service mark.” This symbol is nearly identical to that of the TM symbol, with one exception. This symbol is used in association with services, rather than products or goods.
Takeaway
It is important to understand the differences among (R), TM, and SM. If you are using the wrong symbol, you may be forfeiting certain rights or violating the law.
For example, in a trademark infringement case, a trademark owner can recover lost profits and damages if the trademark owner can establish that the infringer had knowledge of the trademark registration prior to the infringement. Proving actual knowledge can be extremely difficult. However, the trademark owner can establish constructive knowledge if the registration symbol was displayed in connection with the trademark. In other words, the infrigner cannot feign ignorance to avoid a finding of trademark infringement. Thus, you may unintentionally forfeit your ability to recover lost profits and damages if you do not use the registration symbol and can't prove that the infringer knew of your trademark rights.