Nurses are assistants to doctors and/or they take care of patients staying at a hospital. To be a nurse you need an undergraduate degree, but to become a doctor you need to do postgraduate study.
Traditionally, doctors were men and nurses were women, but this distinction no longer applies. The overwhelming majority of nurses ARE still female, but there are male nurses. I think the proportion of male and female doctors are essentially the same nowadays.
A medic is a doctor in the military. Their job is to treat injured soldiers.
A paramedic is someone whose job is to treat the injured at the scene of the accident. If you get into a car crash, paramedics might arrive and try to help you. If you're badly injured the paramedics might ride with you in an ambulance, trying to help you while driving. When you arrive at the hospital the paramedics hand you over to the doctors who work there. I'm not sure if paramedics are technically doctors or not.