MA: Medical Assistant. You can really only work in a doctor's office with this certification, and you usually make significantly less than a nurse. They are NOT a nurse, and they work under the license of the physician. You do not have to go to school to do this; it can be on the job training. So that person doing your xray or giving your kids that immunization in the ped office? Could be licensed, or could be taught on the go. In my facility's doc offices, MAs start at $10/hr (just a dollar or so higher than a certified nurse's aide).
LPN/LVN: Licensed practical nurse/licensed vocational nurse. This is the lowest tier of licensed nursing. These nurses have typically completed a one year nursing education (after prerequisites, which usually take a year or two to complete and include basic college prereqs, biology, and A&P), and they have passed their state licensing exam for LPN/LVNs. They must work under an RN--there are meds and procedures, dictated by individual State Boards of Nursing, that specify what an LPN can and cannot do. LPNs at my facility start at $13/hr.
RN: Registered nurse. Has completed two years of nursing education after prereqs, either at the community college or university level, as well as having passed a state licensing exam. Are able to do more meds and procedures than an LPN, and typically supervises LPN care. RNs at my facility start at $21/hr.
I work in a hospital. MAs are only hired by the facility to work in the physician offices. LPNs also work in the physician's offices (they are kind of interchangeable with MAs in that particular environment), and we have limited numbers of them in our hospital. In my hospital, LPNs are used as scrub techs in surgery or used out on the floor as a nurse. LPNs work on a team with an RN and they may not give IV medications, unless they have completed an additional 40 hour course. Then they are limited to certain meds--they can give pain meds, most abx, and diuretics into peripheral lines only. LPNs cannot give blood or do anything with a central line site in my state. RNs have greater responsibility for "managing" the pt's care, and for documentation. LPNs are used in our ER, our medical floor, and our skilled rehab floor. They are used in a very limited manner on our OB/peds floor to enter orders, scrub for c-sections, stock the unit, and provide pt care. They are, however, phasing them out on OB--they aren't firing the ones they have, but they aren't hiring any new ones. Most floors (except the skilled rehab floor and ER) are not hiring new LPNs. Our ICU has never hired them. LPNs will continue to be able to work in our hospital in the ER, on the skilled rehab unit, and in the physician offices. The medical/surgical floors will only be hiring LPNs who are in school to get their RN.
Hope this helps.