Link Monsters are a type of Monster Card. The first Link Monster was released in Code of the Duelist. These are blue Cards with a hexagonal pattern on them to separate them from Ritual Monsters. They are placed in the Extra Deck.
Properly Special Summoned
To be properly Special Summoned, Link Monsters must first be Link Summoned. If they leave the Extra Deck without being Link Summoned, they cannot be Special Summoned.
Cannot exist in the Hand
Like other Extra Deck Monsters, Link Monsters cannot exist in the hand or the Main Deck. If a Link Monster would be moved to either the hand or the Main Deck, it is returned to the Extra Deck instead.
Link Monsters have a Link Rating in place of a Level or Rank. A Link Monster's Link Rating determines the total number of Link Materials required to Link Summon it. A Link Monster's Link Rating is also equal to the number of Link Arrows it has. A Link Monster's Link Rating can be found next to its ATK in place of the traditional DEF value.
No Level or Rank
Link Monsters do not possess Levels or Ranks, therefore, they cannot be used as material for a Ritual, Synchro, or Xyz Summon. Also, Level or Rank-modifying Cards such as "Harmonic Waves" do not affect Link Monsters. The same can be said for Spells/Traps that affect Levels/Ranks such as "Gravity Bind".
Only in Attack Position
Link Monsters can only exist in face-up attack position (they do not have a DEF value) and cannot be placed in or changed to defense position (either manually or through a Card Effect); this is because rotating the Card into defense position would change the directions its Link Arrows point to. Because of this, Link Monsters are unaffected by Effects that change battle positions and are unable to be placed face-down, and cannot be Summoned by an Effect that would Special Summon Monsters only in Defense Position (such as "Back to the Front").
Link Monsters have Link Arrows, indicated by the red arrows pointing outward from the artwork. These arrows show which Zones and Monsters that Link Monster points to.
Monsters in the Extra Deck (Fusion, Synchro, XYZ, face-up Pendulum, and Link Monsters) can be Special Summoned from the Extra Deck to a Main Monster Zone a Link Monster points to. If a Link Monster points to a Monster, the Monsters are considered to be "linked" to each other.
If two Link Monsters point to each other they are considered “co-linked”
A Link Summon is the act of Summoning a Link Monster from the Extra Deck using the required number of Link Materials listed on its Link Rating. During your Main Phase, you can Link Summon a Link Monster by sending the face-up Link Materials from your field to the Graveyard.
Normally, you can only use one Extra Monster Zone at a time. But you can create an Extra Link which means you can use both Extra Monster Zones.