Source: Yugioh Wikia
An Extra Link is a series of co-linked Link Monsters that connects one Extra Monster Zone to the other. The monsters that make up the Extra Link are considered to be Extra Linked; additionally, any monster co-linked to an Extra Linked monster is also Extra Linked. An Extra Link is formed regardless of which player controls the monsters, including the monsters in the Extra Monster Zones.
An Extra Link can also be accomplished with fewer monsters with co-linked monsters pointing diagonally. If Link Summoning a Link Monster in an Extra Monster Zone would create an Extra Link, the player can do so even if they already control a monster in the other Extra Monster Zone; this is the only way for a player to control monsters in both Extra Monster Zones