Controller

Definition in the Law art. 4

the natural or legal person, public authority, agency or other body which, alone or jointly with others, determines the purposes and means of the processing of personal data. Where the purposes and means of processing are determined by normative acts, the controller or the specific criteria for its designation are laid down in those normative acts;

In plain words

The controller is whoever determines the purposes and means of processing, alone or jointly with others. A company that decides which customer data to collect and why is the controller of that data.