When you’re looking to register an international trademark, realize that some countries don’t allow you to register a trademark unless it is “distinctive.” For example, an application to register a single letter or a short combination of letters and numbers may be refused registration in some countries because it is not “distinctive.” Trademarks in the US don’t use this terminology, and it’s not uncommon for short a combination of letters and/or numbers to be registered in the US.
Trademarks in the US are generally assumed to be distinctive unless they are generic or descriptive. (I’m not talking in detail here about the different kinds of descriptiveness, like geographic descriptiveness.)
A generic mark is a mark that is a general term for the goods or services for which you seek registration. For example, you cannot register FRUIT as a trademark for apples.
A descriptive mark is a mark that describes a feature, target customer, geographic origin, or other aspect of the goods or services for which you seek registration. For example, you cannot register RED as a trademark for apples because the mark describes apples—other companies need to be able to use that term to describe their products.
If a US trademark examiner takes the position that a trademark is descriptive or generic, you can submit an argument describing why you don’t think the trademark is descriptive or generic. This usually involves referring to past legal cases from the courts or administrative decisions by the Trademark Trial and Appeals Board (TTAB).
A generic trademark cannot be registered in the US.
A trademark that is descriptive can be registered only if it has acquired distinctiveness or has secondary meaning in the minds of customers. That is, customers who see the mark must think of the mark as identifying the source of goods or services (e.g., your company), rather than only as describing those goods or services.
If only a part of your mark is descriptive, then you may be required to disclaim that part of the mark. This means that others would be free to use that part of your trademark in their trademarks. For example, if you wanted to register WATER OF THE VALLEY for bottled water, you would be required to disclaim the term WATER within your mark.
When a part of a trademark is disclaimed, the mark itself is not changed. The disclaimer is only a statement within the application that means that you are admitting that part of the mark is descriptive and is available for others to use on similar goods and services. You’re giving up your exclusive rights to the portion of your trademark that is disclaimed.