In the gay slang, twink is a term that describes a young or young-looking male, usually of slender build, only slightly muscular, with little to no body hair. Often they are described as blond-haired and blue eyed. To many gay men, the term is pejorative and implies shallowness and stupidity. Twinks are typically contrasted with bears.
"Twink" may also be used as a generic derogatory term to describe a weak or effeminate male.
In internet gaming communities, to "twink" is to equip a new character with equipment, status etc. not usually available to them by having another player give it to them. See twinking. In tabletop role-playing communities, a "twink" is a slightly less pejorative form of the term "munchkin," indicating a younger player with more interest in their character as a collection of statistics than as a fictional person.
The ultimate etymology of the word--where did it come from? which meaning came first?--is something that really interests me.