There is a huge difference.

RN's are Registered Nurses.  They hold a license to practice registered nursing, and are graduates of either a four year Bachelor of Science in Nursing (BSN) program, or an Associate Degree/Associate Science program that theoretically takes 2 years but can take longer for those who complete required courses before starting the core nursing courses.

RN's can perform full physical assessments, administer all sorts of medications, perform complicated skills such as IV insertions, catheter insertion, wound care and so on.  They are trained in managing the care of patients with complicated pathophysiological processes.  They can tell when a patient is doing better or getting worse, and they know why.  They can plan and implement care, and evaluate the effectiveness of care.  They work closely with physicians on care issues.

CNA's are Certified Nursing Assistants.  They are not nurses, and may not call themselves nurses.  They receive short term training, often in a continuing education program at a community college or other small school.  The training may last anywhere from 8-16 weeks.  They are certifed by the state, and must be supervised by a nurse in all things.  Some CNA's are trained to perform advanced level skills such as putting in foley catheters, but as a general rule they don't do anything involving sterile equipment.

Experienced CNAs may recognize if a patient is getting better or worse, but they can't explain why.  They are not involved in the planning of care.  They can do some nursing tasks such as bathing, changing bed linens, ambulating patients, assisting them to eat, to keep clean and other activities of daily living.  They may assist physicians in minor tasks but do not work with doctors in planning patient care.  They can obtain vital signs, but not do a physical assessment, plan care, or evaluate the effectiveness of care.  They do not give medications.