A "bust," in NFL parlance, is a player whose performance in the NFL is not commensurate with the level at which he is drafted. Rather, it is far below that level. Different positions and different picks have different expectations; for example, more is expected of the No. 1 overall pick than the No. 32 pick, even though both are first-rounders. Either may be characterized as a "first-round bust" if those expectations are not met, although the consequences for the franchise tend to be less disastrous when the "bust" pick was made lower in the draft.
In my opinion, there are five basic types of busts in the NFL; that is, there are five main reasons why highly-drafted prospects fail. They are chronic injuries, lack of desire/motivation, lousy character, being physically unable to perform and being mentally unable to perform at a NFL level.
1. Injuries. This prospect may have all the talent in the world, all the physical and mental aptitude you could ask for, but it doesn't matter because you never see it on the field. Some first-round picks spend their entire (brief) careers with the team in the training room. Their injuries may be chronic, like knee or back problems (think Andrew Bogut), or they may simply be prone to freak injuries (Mike Neal). Sometimes the problem is visible in college, and sometimes it just appears after the prospect enters the pros.
All-NFL Example: Greg Cook, the No. 5 pick in 1969, looked and played like Joe Montana and seemed like he'd lead the Bengals for a decade. But after a torn rotator cuff (initially undiagnosed) in just his third game, Cook's career rapidly fell in the toilet. To this day, he is remembered as the greatest NFL QB that never was.
Packers Pick: Justin Harrell. The 2007 first-rounder hung around for four years, but chronic back problems and an ACL tear prevented him from ever getting on the field.
2. Lack of Desire. Some players may have plenty of physical talent, but no desire to apply it on a NFL field. Maybe they don't actually like playing the game, maybe they feel like there's no reason to play hard after getting signed to a huge contract, maybe they're too reliant on their innate physical talents and don't feel a need to work hard to improve. Whatever it is, these prospects just won't bring it when they're supposed to, and tend to get cut from their teams pretty soon because of it.
All-NFL Example: JaMarcus Russell. Easy choice. The former No. 1 overall pick could throw the ball 50 yards without setting his feet, but completely didn't care about playing football. One of the biggest busts in NFL history, Russell was so bad that the Raiders cut him after three years, then tried to sue him to reclaim some of the record contract they paid him. Nope.
Packers Pick: Tony Manderich. Four of the top 5 picks in the 1989 NFL draft are now Hall of Famers. Manderich, the Packers' choice at No. 2, isn't. Gifted with incredible size, speed and strength, Manderich just didn't have anything resembling a work ethic. He lasted three years before being unceremoniously cut.
3. Lousy Character. Closely related to #2, this category is for those high picks who were either terrible teammates or had multiple run-ins with the law. Some played well, some poorly, but all earned their way out of town based on character issues. Those could be anything from arguments with the coach to domestic violence to substance abuse.
All-NFL Example: Todd Marinovich. The 1991 24th overall draft choice was raised and tutored by his father to make him the perfect quarterback, and it nearly drove him crazy. He started using drugs in high school to deal with the stress and continued throughout college and the pros. He failed three NFL drug tests while with the Raiders and was soon ejected from the NFL, never to return.
All-NFL Example II: Adam "Pacman" Jones. Marinovich, by all accounts, wasn't a bad guy. Jones, by all accounts, is. Drafted sixth overall in '05 by the Titans, Jones had amassed a laundry list of criminal charges and served a year's suspension from the league by 2008. He was traded to the Cowboys, then released after more altercations with the law, and is currently a Cincinnati Bengal.
Packers Pick: Ahmad Carroll. The immature 2004 first-round cornerback constantly drew pass interference and illegal contact penalties, refusing his coaches' entreaties to play within the rules. After a particularly lousy game in 2006, he was unceremoniously cut.
4. Physically Unable to Perform. There are certain body types--quarterbacks under six feet tall, offensive linemen over 350 pounds, etc--that can work in college but just don't translate to the NFL. This is a particular problem with "tweeners", defensive ends that are too light to play that position in the pros and thus project as outside linebackers. Through no fault of the prospect's own, he finds himself just physically unable to do what the NFL requires he do at that position.
All-NFL Example: Aaron Maybin. Maybe the most egregious example of this category, Maybin was drafted 11th overall in 2009 by Buffalo. At 6'4", 228 pounds, Maybin was far too light to play defensive end in the Bills' 4-3 defense (those ends are typically 260-275 pounds). The Bills switched to a 3-4 in 2010, but Maybin couldn't play outside linebacker either, and was let go after that season.
Packers Pick: Rich Campbell. The 1981 sixth overall pick simply didn't have the arm strength to be a NFL QB, as the Packers discovered, and never made it on the field for an extended period of time. In four years with Green Bay, he threw for 386 yards (no, that isn't a typo) with three TDs and nine picks. Total.
5. Mentally Unable to Perform. This category is the hardest to understand. Prospects in this category may be model teammates, have the best physical measurables possible, have all the heart and desire and want-to of the best players in the game, but simply can't make it work mentally on the field. Often, these players' draft position is inflated due to their great measurables, and they earn the title of "workout warrior".
All-NFL Example: Vernon Gholston. Gholston, picked No. 6 by the Jets in 2008, was the prototypical workout warrior. He had one of the great combine workouts in NFL combine history, and had put up tremendous numbers in his third year at Ohio State, but they never translated to the NFL. (Gholston was also expected to play OLB, making him a candidate for category #4 as well.)
Packers Pick: A.J. Hawk. Hawk isn't generally labeled a "bust", but few would argue that he's played up to the level that No. 5 overall draft picks are supposed to. Gifted with all the measurables you could want, this former Buckeye just doesn't possess the instinctiveness or the willingness to hit people generally found among the NFL's best middle linebackers. He has all the physical tools to play the game at a high level, bur for whatever reason, has never done so.