Alright, kids, let's have a true, unbiased, comprehensible, detailed definition. First off, loners are not necessarily social retards or depressed. Yes, one could be a loner because they are depressed or lack social skills, but there are also many loners who are named as such because they simply prefer their own company. Loners are characterized by enjoying solitary activities, such as listening to and making music, constructing art, writing, reading, thinking, learning, gaming on the internet and in video games, going on the computer for whatever reason, and other individual hobbies. Loners listen to whatever music they like; anywhere from Baroque to Blues Rock, Heavy Metal to Folk, Indie to Rap, Country to Techno. They dress however they please; they may have their own individual style, or just wear standard jeans and a t-shirt from whichever store at which they find themselves. Loners are just as likely to suffer and not suffer from depression and other mental disorders as anyone else; however, more commonly found amongst loners than those that are social may be Schizoid Personality Disorder
, because it is, by definition, a "disorder" in which one lacks the desire for interpersonal relationships. Society today associates itself with a dogma that everyone desires to be socialized, have a close-knit or large group of friends and loved ones, etc, so that when there is someone who prefers solitude, they are often attributed as strange, depressed, feeling unloved, or insecure. This is not true. Loners are very diverse individuals; same as a case with someone you may find who is a social butterfly that fits into no specific group, you cannot stereotype or pigeon-hole loners. The only characteristic they possess that detaches them from another label is that they prefer their lonesome company.
Loners just like to be by themselves. That's all there is to it.