One of the oldest scams around, and this one has been around for at least 10 years. They have reduced the amount they rip people off for recently, and the wording on the letter has changed, but still the purpose is to get you to change your registrar when all you want to do is renew the domain.
They will email or post you a letter, around the time it is due for renewal.
The letter looks authentic, the print quality is good, the details are usually correct, and there is enough real information to fool most people. Just throw them away when they come. Here is what they look like