Dark Link is Link's evil doppelganger the Zelda series. Despite whatever game he's in, he is always a dark contrast to Link himself. Being total equals in size, strength, and skill.
His first appareance was at the end of Zelda II: The Adventure of Link. I never made it that far, so I missed it.
The first time I ever saw Dark Link was his mini-boss appearance in Ocarina of Time. It was pretty rad.
Dark Link then got pushed mainly to the portable games.
I like the Four Swords Dark Links.
Not so much the Dark Links in Twilight Princess. CREEPY...