There is only one difference between registry studies and clinical studies: registry studies are observational and clinical studies are investigational. (When clinical studies are randomized they are called randomized clinical studies or RCTs.) To put it another way, in a registry study we tell the physician to treat the condition however they want—as sponsors, we are passive observers; in a clinical study we instruct the investigator to treat the condition in a certain manner—we are active researchers.
There is another, inevitable, feature of registry studies that I want to point out. The words "effectiveness" and "efficacy" are often misused, even by FDA. Effectiveness refers to how well a device performs as intended in the general population of patients and the general chaos of clinical practice. Effectiveness is measured in registry studies. Efficacy refers to how well a device performs in a setting of carefully selected patients and a carefully controlled protocol. Efficacy is measured in clinical studies.