A data controller is the entity/person that determines purposes and means of processing personal data of the EU resident. For eg. Tesloop is a data processor and Tesloop customers are controllers of the EU resident's data.
The GDPR applies to both data controllers and processors. Controllers collect data from the end-user that is the EU resident, for purposes clearly stated and with appropriate consent. Data processors provide services to the controller in accordance with each controller's instructions. Processors also use data collected to perform benchmarking analysis, so that it can sell further services allowing controllers to compare their data to industry averages.
Another category called sub-processors or third-party businesses performing data processing for other companies are also accountable for protection of personal data, according to the GDPR