I coordinate the Extern Program at my hospital -- and have done a lot of reading on the subject, etc.
The above posters are correct. It really does vary from place to place. People usually use the term to describe special roles created for nursing students, but the exact nature of those roles varies from hospital to hospital. Also, people tend to use different terms to describe similar roles.
Therefore, my recommendation is to call the hospitals that you might be interested in working at and ask to speak with the Nurse Recruiter. (She's the one who probably has the most information on the subject.) Ask the Nurse Recruiter, "What roles do you have at your hospital that would be suitable for a nursing student?" If you just ask about an "extern" role and they don't use that term, you might miss out on a good opportunity for a role with a similar function, but a diffferent name.
At my hospital, we have a couple of roles that are well-suited for nursing students and are in the process of creating a couple more. Each role has a different name, different duties, different requirements in terms of education and previous experience, different scheduling options, and different pay. Some are more clerical in nature or focus more on equipment maintanence, stocking supplies, etc. A beginning student who is not yet qualified for direct-patient care roles might like such a role as a chance to at least work in a hospital environment. We also have a role for students with at least 2 semesters of clinical under their belt. Those students can do a lot of direct patient care, including starting IV's and draw labwork as well as gather vital sign data and provide basic hygiene, etc. As a student looking for a job, you want to learn about ALL of the options. So, be sure to ask your questions in such a way that you will discover ALL the options.
By the way ... At my hospital (a Children's Hospital), we use the term Extern to refer to a special program we run only in the summer -- not for a job that runs all year. Our Extern Program is a full time summer job for nursing students who have completed at least 2 semesters of clinical coursework in school. The Externs are always paired with an RN and the 2 people provide complete care for a "1-person assignment." The Extern is allowed to participate in any and all clinical procedures as long as the RN is physically present and actively supervising the procedure. Without the RN physically present in the room, the Extern can perform CNA-type skills.
As our staff works 12-hour shifts, 3 shifts per week equals 36 hours. So, we also pay the Externs for 4 hours of educational time per week -- IF they want it. Some weeks, we have organized classes that the Externs are required to attend. Other weeks, the Externs are free to explore their own educational interests on paid time. Most choose to use that time observing care on other units. Except for the organized class time, that 4 hours of educational time is optional and most Externs choose not to take advantage of it each and every week. They can use it to fulfill their goals, but they don't have to work those 4 hours if they have something else they would rather do. We have no requirements for them after the summer program is over.
Good luck finding a role that is right for you,