Most college football players are classified by their class year. Freshmen, sophomores, juniors and seniors.

But some are redshirt freshmen, sophomores, juniors or seniors. And then there are grayshirts and other terms used to define other eligibility and recruiting statuses.

Here's a quick guide to what each of these terms means, with some more explanation below.

Shirt type On scholarship? Can play? Can practice? Meaning Redshirt Yes No Yes Athlete sits from competition for a year, then gets another year to complete four seasons. Grayshirt No No No Athlete waits an extra semester to become a full-time student and part of the team. Blueshirt Yes No Yes Athlete is not formally recruited, but is put on scholarship at start of freshman practice. Greenshirt Yes Yes Yes Athlete enrolls in school a semester early.

This is the most common. A redshirt year is a year in which a player is on scholarship and can practice, but does not play, in exchange for an extra year of eligibility. If a player redshirts — meaning they don't play for one year — they will have five years to compete in four seasons.

The term comes from a Nebraska player in the 1940s who chose to only practice, not play, as a sophomore, in order to catch up physically. He wore a red jersey with no numbers on it ... a red shirt. He's the oldest example of a player intentionally held out of a season for this reason. The names of these other 'shirt terms spring from redshirting.

Many college football players redshirt their freshmen years in order to get physically ready. Other players redshirt due to injuries. These are the rules a player must follow in order to obtain a medical redshirt:

The injury must be incapacitating, a season-ending injury.

The injury must occur prior to the start of the second half of the season.

The student-athlete must not have competed in more than 30 percent of the season or three contests, whichever is greater.

The NCAA has also added an academic redshirt this year. Players will be forced to redshirt their freshmen years if they don't meet certain academic requirements from high school.

This often comes up in recruiting around National Signing Day. A grayshirt is when a team offers a player enrollment on scholarship at the start of the second semester, after the upcoming season. The athlete then has five years to play four seasons, with the ability to redshirt at some point.

Athletes who grayshirt are allowed to enroll as students. They go to class for the first semester as part-time students, either at the school or at a junior college, without starting their eligibility clocks. Then they begin as full-time students on scholarship.

Grayshirts have commonly been used by programs that oversign, which means not having enough roster spots for all of their commits. Sometimes, coaches are up front about grayshirting from the beginning. However, other times, they will force grayshirts on athletes at the last minute, leaving them unable to find schools that will allow them to play immediately.

Last year, Louisville coach Bobby Petrino was criticized for giving one player a surprise grayshirt right before National Signing Day, after the player had been committed as a non-grayshirt for a year. Many other programs have done the same thing.

Sometimes, grayshirts can be revoked and turned into regular offers due to unforeseen roster changes. If there is a surprise transfer during the offseason, a player who planned on grayshirting could be added to the roster as a normal signee.

Blueshirting wasn't used until recently, and it still isn't common, but it's essentially another loophole to get around oversigning.

The blueshirt rule allows schools to put "unrecruited" athletes on scholarship once they arrive on campus, but count them against the next year's scholarship total, as long as they don't play. Here's what it means to be "recruited," according to the NCAA:

Was provided an official visit to the campus;

Had arranged, in-person, off-campus contact with a coach; or

Was sent an NLI or other written scholarship offer.

Coaches can contact players and have them on campus for unofficial visits — when the recruits pay their ways — without technically recruiting them. Essentially, as long as a recruit doesn't take an official visit or host one of that school's coaches, he wasn't "recruited" by that school.

Tennessee has popularized the practice, which was first developed by New Mexico State.

The more common term for a greenshirt is "early enrollee." A greenshirt is a player who graduates high school in December of their senior year and opts to enroll in college before Signing Day. Athletes who greenshirt still have five years to play four seasons, so they can still redshirt.

There are many benefits to greenshirting. These players can get ahead on their classes. They are on full scholarship in the spring and are allowed to participate in spring practice and work out with the team; Alabama used an early enrollee quarterback to prepare for Clemson in this year's National Championship.

Greenshirting has become much more common in recent years, as players are looking to get college-ready quicker.