A trademark is any word, name, symbol, or device used by a person or which a person has a bona fide intention to use in commerce to indicate the source of goods or services and distinguish his or her goods. Trademarks are types of intellectual property and are mostly governed by Federal law. Trademarks may be registered with the United States Patent and Trademark Office (PTO), but still give the trademark holder some rights even when not registered as there are common-law trademark rights to protection in addition to the rights created by Federal law. I'll give you the "quick and easy" version of when to use certain trademark symbols on your various product or service marks.
® is a symbol you can only use when you have registered your mark with the PTO. Why register you ask? There are many benefits, but the most important one being that a person or entity cannot bring a lawsuit in Federal court concerning a mark if that mark is not registered. Registering your mark also grants you prima facie evidence of the validity of your registration and gives you nationwide constructive use of the mark as of the date of your application. The safest time to use this mark is when you have registered your mark with the PTO and received notice back of its validity.
™ is a symbol that many other people should be attaching to marks on products. Trademarks are territorial in nature and the symbol ™ indicates to the reader that the person who placed that mark on that product, is indicating some type of ownership or interest in the mark. It does not mean the trademark claim is valid, but it simply alerts the public that you think you have some rights to that mark. You do not need to pay any money or register your mark in order to place the ™ symbol on your mark and it is not used enough in small businesses.
Service marks are the same as the trademark "™" symbol except that the small T is replaced by a small S to indicate a service rather than a good.
This was the oversimplified course in trademarks. However, with the marketplace being as global as it is, this information is critical to every business owner regardless of the size of one's business.