As the owner of a trademark, when you can stop others from using your trademark, or a confusingly similar one, depends on such factors as:
In addition, under federal (and some state) laws known as dilution statutes, you may go to court to prevent your trademark from being used by someone else if your mark is famous and the other company's use would dilute the mark's strength -- that is, weaken its reputation for quality (called tarnishment) or render it common through overuse in different contexts (called weakening). The key element is that your mark is famous -- that is, distinctive and recognizable.
Dilution statutes apply even if there is no way customers would be likely to confuse the source of the goods or services with those sold by the owner of the famous mark. For instance, consumers might not think that Microsoft toilet paper is associated with Microsoft, the software company, but the makers of Microsoft toilet paper could still be forced to choose another name under federal dilution law.
A business that claims to own a trademark cannot stop others from using the same or a similar trademark unless it is actively using the trademark.
In trademark law, "using" a trademark means putting it to work in the marketplace to identify goods or services. This doesn't mean that the product or service actually has to be sold, as long as it is legitimately offered to the public under the trademark in question.
For example, Robert creates a website where he offers his new invention -- a humane mousetrap -- for sale under the trademark "MiceFree". Even if Robert doesn't sell any traps, he is still "using" the trademark as long as "MiceFree" appears on the traps or on tags attached to them and the traps are ready to be shipped when a sale is made.
Similarly, if Kristin, a probate attorney, puts up a website to offer her services under the service mark Probate Queen, her service mark will be in use as long as she is ready to respond to customer requests for her advice.