There are several different levels of medical care that you can take advantage of. They each have their own appropriate times to be used, and it's important to know when you should use each kind of medical care.
Primary Care Physician
Primary care physicians are who you see for your regular health care. You go see them for things like physicals and routine medical issues. For example, if you have diabetes, you may go see your doctor every few months so that they can keep track of your bloodwork and check your overall health. If you have a concern about some medical issue, you would call their office and either ask your question or schedule an appointment. You usually don't go to see them for an emergent health issue unless you can get into see them that day or the next day.
An urgent care doctor or clinic is somewhere that you can visit if you get sick or injured and you don't want to get into your doctor's office or you can't get into them immediately. For example, if you wake up vomiting and can't keep anything down, you may call your doctor's office and find out that you can't get into see them that day. However, you want to get checked out by a doctor, so you go see an urgent care clinic. You will get into see them immediately and you will be able to get treated. Generally, you wouldn't go see them for something that is immediately life-threatening.
The emergency room is where you go when you have an emergent or life threatening issue. For example, if you were to fall down and break your leg, then you would go to an emergency room. They would have the appropriate equipment to diagnose and treat your broken leg. They would be able to take an x-ray of your leg, rate the severity of the break as well as where the break is, and start treatment by putting on a splint or cast. The emergency department would refer you to a specialist after that to continue the care and treatment of your broken leg.
Knowing which level of medical care you should use when something happens is important. That will make sure that you get the right treatment at the right moment. You don't want to have to wait for treatment or have to go to more than one place when something is happening.