So many people use this term to describe extreme introverts. However, this is not what "antisocial" means.
Antisocial: to work against society. An antisocial individual is particularly selfish, has no regard for others, and has a tendency to step over people on their way to achieving their own personal wishes. An antisocial actually enjoys interacting with other people, for the sole purpose of getting what they want. It is very difficult to con someone if you don't interact with them.
Introvert: an individual who gathers their energy through quiet contemplative, solo activities. Introverts prefer their own company, though they do have friends. The friendships are few, but very meaningful and rich, and valued very much by the introvert. While introverts may socialize with other people, it taxes their reserved energy. They will need quiet time alone to recharge their batteries before venturing out again.
Nonsocial: an individual who wants nothing to do with society. These people either disdain human contact or have extreme anxiety related to social activities or social interaction of any kind.
When most people refer to an "antisocial" individual (e.g. "Why are you going to the party if you are so antisocial?") they are usually thinking of a nonsocial who wants nothing to do with other people. A nonsocial may go to social gatherings, but these are usually the "wallflowers" that you see avoiding eye contact at all costs. An introvert will attempt to engage in conversation with one or two individuals with whom they are familiar and comfortable or, barring this, will actively observe the other people at the gathering. A nonsocial will bury themselves in their cellphones, e-reader, etc. until they can slink away to the solitude of their homes. An antisocial will try to persuade anyone at the party to give them something (usually conning them out of money, favors of a sexual or nonsexual nature, etc.) that they want at that time. The antisocial has no conscience, either, so they may have no qualms about breaking hearts or insulting people at the party. Being good actors, they may be the life of the party or the "perfect" guest until they grow tired of it, then they will move on to new hunting grounds where they can find fresh victims to exploit.