An unstable human being that inevitably crashes horrifically. Trainwrecks are frequently bisexual, have multiple (sometimes ignorant of one another) sexual partners, and a general or total lack of organizational skills. Trainwrecks are typically, though not always, female. They are usually lower class, but upper class trainwrecks are not rare. Lower class trainwrecks typically use sex as an exchange medium. Upper class trainwrecks typically use sex to control people. Both frequently use sex to substitute for interpersonal skills in establishing or maintaining relationships. Almost all trainwrecks have a LONG list of continuous problems and a similar or smaller list of recurring problems. Virtually all trainwrecks have multiple dependents (lovers, children, parasites, etc.) that help speed the trainwreck into the inevitable "shitstorm." Trainwrecks do not improve under their own power. They require an outside force to push them on the path to recovery.
Lindsay Lohan, Britney Spears, and MC Hammer are all of examples of trainwrecks.
Many other celebrities exemplify trainwreck behavior, but are not necessarily trainwrecks.