What guyliner is:
Guyliner is a slang term for eyeliner being worn by a male.
Eyeliner, and other makeup in general, is usually considered a female thing. In "normal" society, it is still generally taboo to see eyeliner on guys, however it is slowly becoming more acceptable as the years go by. Because guyliner is still generally taboo, you will usually only see it on guys trying to buck normal societal trends, or on very confident men, or both. Example types are emos, goths, punks, rockers, certain celebrities & some fictional characters. There's a few entires on Urban Dictionary that claim that homosexuals wear it also, but I haven't personally seen any evidence that homosexuals go out of their way to wear it because of their sexuality. If the claim is true, then perhaps homosexuals are more likely to wear guyliner than heterosexuals because they don't have the normal societal macho male image to maintain and can therefore be more free in their expression of selves.
Famous fictional portrayals of pirates have popularised the use of guyliner for male pirate costumes. Captain Jack Sparrow from the movie franchise Pirates of the Caribbean is arguably the most influential character for guyliner in pirate costumes in recent times. Blackbeard from the fourth film in the same franchise also used guyliner to great effect. At least two Captain Hook characters used guyliner, one from the TV show Once Upon a Time and the other from the movie Hook.
A selection of famous males who wear eyeliner, some more often than others:
Tim Minchin (singer)
Adam Lambert (singer)
Brandon Flowers (The Killers)
Billie Joe Armstrong (Green Day)
Ozzy Osbourne (Black Sabbath)
Boy George (Culture Club)
Alice Cooper (singer)
Freddie Mercury (Queen)
Dave Nevarro (Red Hot Chili Peppers / Jane's Addiction)
Nikki Sixx (Mötley Crüe)
Russell Brand (comedian, actor)
David Bowie (singer)
Johnny Depp (actor)
Jared Leto (30 Seconds to Mars, actor)
Mick Jagger (The Rolling Stones)
Keith Richards (The Rolling Stones)
Steven Tyler (Aerosmith)
Elvis Presley (singer, actor)
What guyliner isn't:
The following may be splitting hairs. For some people, eyeliner on males is guyliner no matter what, and there are no subtle distinctions.
The eyeliner seen on men in early silent movies would not be considered guyliner. Instead, it would be considered general movie make-up. Eye makeup in the first few years of early movies was often overdone, as actors were used to the makeup used on stage. Stage makeup had to compensate for bright lights washing out facial features, and so was (and still is) often overdone on purpose by comparison to normal every day makeup. Eye makeup in the later years of early movies was less overdone but still used, as they needed to compensate for unique technical issues with early black & white film technology.
The eyeliner worn by male ancient egyptians would not be considered guyliner, as eyeliner was worn by both sexes equally as standard in that society.