A person who works his/her butt off preparing for students and trying to help them get a head start in life. They work for little pay, and are spoken poorly about by people ignorant of their cause. As with all people, some care more than others. But they still do more than the average onlooker observes.
A teacher can be found planning lessons, grading papers, researching activities, going to meetings, calling homes, greeting people, tolerating bullcrap, rationalizing, destressing, parenting for parents who don't know how to or don't give a damn about parenting.