A professor evaluation is a way for students to anonymously, and honestly, assess their professor's performance at the end of a semester or year. The professor evaluation is an important aspect of assessing teaching for many colleges, and it also helps professors learn where they need to improve. If you are being asked to complete an evaluation for a university professor, you should take it seriously, and give honest answers -- the evaluations are anonymous.
In most colleges, the same paper professor evaluation is used across the board. This means that students will generally be given the same questionnaire for numerous professors, which is beneficial when it comes to comparing the results. The students also know what to expect ahead of time, and are able to consider their answers in advance.
The questions asked on professor evaluations usually include information about the way class time is spent, the amount of time spent on debate or discussion, the professor's availability to students through office hours or time in class, the amount of homework given, and the time it takes to complete information on tests, papers, or other assignments. Questions on teaching style, and thoroughness in covering material might also be included. Generally, the questions are simply intended to give the professor and his or her superiors a realistic picture of his teaching style, the areas in which he excels, and the areas he needs to improve.
In addition to a professor evaluation that is completed at the end of the semester, and sanctioned by the university, there are numerous websites that encourage students to anonymously evaluate their professors and post reviews of classes. Some of the questions are similar, though others are more geared toward helping the student have an "easy" semester with an appropriately attractive professor. The true anonymity of the web generally produces what might be considered more "honest" results, though some argue that these sites are often dominated by students who received a poor grade and are looking to get revenge on a professor. Many professors check these websites, however, and if enough students leave similar comments, it might encourage them to change their teaching style a bit.
If you have had a particularly negative experience with a professor, and this seems out of the ordinary, it might be helpful to go directly to the professor and discuss your experiences, and ways in which you might have done better as a student. Not only is this a more mature way of handling things than simply filling out a negative professor evaluation, it might help you salvage the relationship with the professor. Keep in mind, you might need to take another class with him or her in the future.