TM stands for a trademark symbol. This symbol is what companies use on a word, logo, phrase, or name. The symbol represents the business and helps customers identify it easily. The registered symbol is often abbreviated with the letter R. It differs because it has a legal standing, while TM's do not. The reason for this is that R's are trademarks that are registered with the USPTO, which stands for the United States Patent and Trademark Office. Anyone can use a trademark symbol, without actually registering it. However, if someone creates a TM that comes to find out is R by another person, the person who has the R has legal standing to sue the person who has the TM.
Just in case you are confused, you should first know that TM means a trademark. TM is said to be confusing for some people at times. This can refer to the different marks that are available that may be registered or not. Usually, you can use TM if your brand or what you are offering still has not been registered for a patent.
This means that you are trying to protect what you have even if it has not been patented yet. Of course, when you have already registered something and it has been approved, you can definitely use R. Take note that using TM does not necessarily mean that your product or brand will be protected.