Professors and teachers with tenure have received a contract that essentially guarantees them a job for life. They can be fired only if they commit one of the three I's:
  1. Immorality (for example, sleeping with a student)
  2. Incapability (contracting a serious illness that makes teaching or research impossible)
  3. Incompetence/Insubordination (refusing to perform assigned duties)

The criteria for tenure vary from institution to institution. Usually, a candidate for tenure must spend five to ten years in an entry-level position, called a tenure-track job, before s/he is eligible for tenure (although if a professor has tenure at one university, he will usually be offered tenure if he applies for a job at another). At the end of this term of service, the candidate is evaluated by a tenure committee that evaluates his/her teaching abilities, service to the university, and--above all--research publications.

Tenure allows professors to research and publish whatever they want without fear of retribution. Without it, administrators and department chairs could use the threat of dismissal to suppress certain lines of inquiry, force a subordinate to pursue research on a particular topic, or simply get rid of a professor with whom they disagreed. Research universities devote themselves to the quest for truth but are nonetheless rife with vehement disagreements, petty bickering, and outright intellectual terrorism; tenure supports the former by protecting professors against the latter.

Although tenure provides researchers with important protection, it's of debatable value in a non-research institution such as a high school or a small liberal-arts college. Tenure does protect teachers from foolish principals who want to force everyone to adopt the latest asinine educational philosophy; however, it also allows lazy, uninspiring, and inept teachers to leech off the system forever.

Furthermore, tenure has its price even at a research university. No system of selection is perfect, so every university ends up with some deadwood--that is, professors who are unproductive. Most such professors dance at the borders of incompetence and insubordination, so the university is stuck with them until they retire.