Excused absence policies vary based on college and the teacher of the class. Each teacher typically has a syllabus that states the policy on excused absences, detailing how to get your absence excused and what is considered excused. Likewise, the policy may contain rules, such as, "You are allowed only two excused absences a semester," meaning there will be penalties if you exceed this amount. This teacher's policy may be purely her own or may be influenced by the university or college's absence policy.
One of the most common reasons an absence is excused is due to religious observances. Holidays, celebrations and other important events related to your religion are typically criteria for getting an absence excused for religious reasons. If you know you'll be missing class for a religious observance, talk with the professor near the beginning of the semester and let her know.
Absences due to university-sanctioned activities are usually considered excused absences. A common example is playing for one of the school's sports teams. Other examples include participating in the marching band, school government and the debate team. Like absences due to religious observances, it's best to check with the professor early in the semester about the upcoming absence(s) to make sure it'll be considered excused.
Some colleges and teachers excuse absences due to illness. Sick people in the classroom can make other students sick. Some teachers require a doctor's note or other proof of sickness in order to excuse the absence. Requiring proof deters people from faking illnesses anytime they don't want to attend class. In addition, other circumstances, such as funerals and family emergencies, may be considered excused. As always, check with the professor to learn what counts as excused and if anything needs to be done on your part to get the absence excused.