In education, teachers in school are those who lecture (US) or teach (UK), give exams and grade students. Teachers in college are called instructors or lecturers.
A teacher who registers a student, or who is positioned to help the student in particular is called a "tutor." A teacher or trainer from whom a student learns a great deal may be called a "mentor."
The term "professor" is usually applied (in the US) to college or university teachers that have received tenure, although there are rankings from Associate Professor through Full Professor that may be defined differently at various institutions.
Teachers who look after the whole school are called "headteachers." The equivalent in colleges and universities—principals—don't teach. A teacher in a grammar or public school in Britain may also be a Head of House. Houses were also used in secondary and comprehensive schools.
Teachers are usually educated in universities or colleges, and must be certified by a government body to teach in public (U.S.) schools. In the UK, teachers in state schools must have Qualified Teacher Status, either through having completed a first degree (such as a BA, BSc or BEd) that leads to this, through completing a PGCE after their first degree, or through on-the-job training at a school.