Depends what you mean by CMA. In most cases it means Certified Medical Assistant, but at the place where I work, our Medication Aides wear name tags with "CMA."
CMA are usually found in doctor's offices. They do simple procedures, take patient history, and usually do vital signs. It costs much more to become a CMA than it does to be a CNA. I'm talking thousands of dollars, unless a community college in your area offers a CMA program.
CNA are found in inpatient centers like hospitals, skilled nursing units, nursing homes, etc. We can be cross-trained (although I do NOT like that at all; I think it's borderline illegal and compromises patient safety). As a CNA I help people with activities of daily living, i.e. things they can't/are limited in doing for themselves. I give people showers, feed them, change their briefs or clothing, take them to the restroom, and transfer them in/out of bed. I also take vital signs, and I report abnormal behavior or conditions to the nurse.