Teachers are professionals who educate students in either an individual or classroom setting. Teachers must be certified with a Teaching degree, and are often also specialized in their particular field that they plan on teaching. Most teaching programs in the United States are formed in order to accomadate this, and are tradionally 5-year college tracks. Teachers can be employees of the government, if working for a public school or other government-operated organization. Teachers are also employed by private schools and other paid educational services. Teachers, however, must be distinguished from tutors in that a teacher's purpose is to introduce and educate students on a subject, wheras a tutor's purpose is to assist students with already-learned material. Teachers work with all age levels of students, and those entering the teaching profession often choose a certain age group with which they would like to teach. Teachers of younger age groups, such as elementary students, often have a broader and more basic knowledge requirement that spans many fields of study; as the student's age increases, teachers tend to become more specialized. Teachers of college students are known as Professors; being seleccted for a Professor position is held in high esteem and often requires a large amount of education and experience not only in teaching but in one's field.