Eric's answer is correct, but I will warn you that most of what he described is very geographically dependent. In Fort Collins and its surrounding communities, 100% of 911 ambulance responses are done by private companies, with fire departments just responding as support. As you move south towards Boulder, more of the Fire Departments have their own ambulances, and the Private Ambulances mostly do interfacility (hospital to hospital) transfers. I've also worked in parts of the country where the fire department doesn't respond to medical calls at all, and private ambulances do everything.
Just like response models change, so do protocols (what skills an EMT is or is not allowed to perform, according to state law and their supervising doctor's orders). In some places EMTs can do very little, but in others EMTs are trusted to do much more. Regardless of what you work you find as an EMT, I would encourage you to think of it as a stepping stone on your way to something else - such as being a Paramedic, a Firefighter / EMT, or Nursing / Med School.
As far as what life as an EMT looks like, there will be many similarities whether you work on an ambulance, fire engine, or an ER. Your day will begin with equipment and supply checks, ensuring you have everything you need and know where everything is because an emergency could happen at any moment. The rest of your day will be spent treating patients and writing reports about the treatment you provided. If it's a slow day and you don't have a lot of patients, you'll probably spend your time studying to keep your skills and certifications up or cleaning and maintaining your equipment and work area. It's not always glamorous, but it's important, and a few hours of boredom is usually followed by a few minutes of terror!
Two medical students talk about their experiences as an Emergency Medical Technician (EMT) and how it prepared them for medical school. You can read their views on it at:
Also, visit the link below on general guide to everything you need to know about EMT