All athletes are given an opportunity to take what is called a redshirt. There are two kinds of redshirting.
The first kind of redshirting is used most often with the freshmen class. Typically male athletes chose to take the redshirt to spend an entire year in the gym lifting and getting stronger. To take this kind of redshirt the athlete declares before the season, and they do not compete in games. The athlete continues to practice and workout with the team, but typically does not travel to out of town games, and does not dress for competition.
The second kind of redshirting is not technically a redshirt. The official terminology for this is a "hardship waiver," This refers to a student-athlete who is seeking to gain an extra year of eligibility because of an injury suffered. At the Division I level, the student can apply for the waiver if the season-ending injury occurs in the first half of the season. The student-athlete must not have participated in more than 20% of the school's schedules contests.
To be granted this waiver takes a lot of work. It requires documentation from all doctor's visits, surgery reports, x-rays, MRI's, and check ups. It is then submitted to the NCAA and a committee goes over all the paperwork to be approved.
If the waiver is granted, the athlete is given an extra year of eligibility.
Taking a redshirt is not an easy way out of a season. I was granted a hardship waiver for the 2008-2009 season. Between the surgery, rehab, and extra workouts I put more time into the season than the people who were actually playing.
Taking a redshirt can also be a struggle mentally for a lot of athletes. They are plagued with the feeling that they are putting all the time into their sport and getting nothing in return.