A written warning (Abmahnung) is a formal request to stop doing something. It's similar to a cease and desist letter, a letter of demand or a mise en demeure. The Abmahnung is often the first step before taking someone to court. It's an official warning.

Anyone can send an Abmahnung. You can send an Abmahnung for something that does not affect you. An Abmahnung can also ask the person to pay for legal costs. For example, if you hire a lawyer to write an Abmahnung, it can ask the person to pay for your lawyer.

Some law firms send them to make money. They target people who pirate movies in Germany, or websites that break minor laws1. They know that people will pay instead of fighting them.