I'm not sure what you mean by certified nurse because there are different types of certification for nursing. You can think of nurses are falling into 3 main types:
- Licensed Practical Nurses, also called Licensed Vocational Nurses: these nurses study an extra year after high school and become licensed by the state they work in. These nurses take medical histories, record patient symptoms, weigh and measure patients. These nurses are often supervised by Registered Nurses and sometimes directly by doctors.
- Registered Nurses: these nurses have a two-year, associates degree, or a four-year bachelor's degree. They must pass a national exam to get their license. These nurses administer some treatments, help patients manage their treatment plans, explain medical information and prevention strategies to patient, and coordinating care with the families of patients. Some registered nurses advance their careers by going into specialties--for example, certified registered nurse anesthetists, cardiology nurses, etc.
- Nurse Practitioners: these nurses get a master's degree in addition to their bachelor's degrees in nursing. These nurses have more autonomy than any others--may diagnose and treat patients. This includes writing prescriptions, running medical tests, and providing basic care.
Hope this answers your question!