Depending on context, a scrub will *usually* refer to an inferior or lesser person in some way. It should be noted that unlike other commonly used slurs such as nigger, redneck, Jew, faggot or whore, the term scrub is a broad encompassing definition that can be applied to individuals regardless of race, gender, sexual orientation or religious affiliation.
With that being said, a "scrub" usually refers to an individual that chooses to live a certain lifestyle that is… "frowned upon." First and foremost, a scrub is deadbeat, and more often than not will have no car, money or job. They are often 100% dependent on charity (family, welfare, etc), and are notorious loners and drug users. They are typically unattractive, have no social lives, and fail in all aspects when it comes to their pursuit of romantic interests
Scrubs typically aren't enrolled in school (as this implies that they're actively working for their future), but if by some off-chance they are then they're undoubtedly failing miserably. So, in conclusion, a scrub is a socially rejected loser who is going no where, doing nothing and failing miserably at all aspects of life.
Allen in two and a half men is a close example for a scrub. Although he does have a car and job, he's still more often than not broke, has horrible luck with women (he admittedly resorts to prostitutes and magazines), and is solely reliant on his older brother Charlie for food, rent and other miscellaneous living expenses.