Data processors are the companies that provide services to data controllers. They can be any kind of business or organization, with the exception of government bodies.
The main difference between a data processor and a data controller is that the former does not have direct responsibility for the processing of personal data. Data processors must work under instructions from their clients, who are responsible for complying with GDPR requirements.
Data processors may process personal data on behalf of another business (a controller) or on their own behalf. They can be instructed by the controller or have their own legitimate interests (such as marketing).
If you are a data processor, you must implement appropriate technical and organizational measures to ensure compliance with GDPR requirements.