The Main Deck, usually referred to as the Deck, is a pile of Cards that a Duelist can draw Cards from. Each Duelist uses his or her own Main Deck in a Duel.
The number of Cards in the Main Deck is public knowledge.
When constructing a Deck, Monster Cards (Normal, Effect, Ritual, and Pendulum), Spell, and Trap Cards can be placed in the Main Deck. Fusion, Synchro, Xyz, and Link Monsters are placed in the Extra Deck. Tokens are to be stored in a separate place.
During a Duel
When a Duel begins, you have to place your Main Deck face-down in the Deck Zone of the field. All Cards in the Main Deck must remain face-down during the Duel, and neither player may view the contents of either player's Deck unless a Card Effect allows them to, after which the Deck must be shuffled.
Even if a Card in the Deck is face-up, it cannot be looked at by either player unless it is on top of the Deck, in which case it is public knowledge. Therefore, all other Cards are not public knowledge.
Main Deck Construction
The Main Deck must contain only Legal Cards.
The Main Deck must contain 40 to 60 Cards.
Only the Cards in the Main Deck, not Cards in the Extra Deck, count toward this total.
You may not have more than three copies of any Unlimited Card in your Main Deck, Side Deck, and Extra Deck combined.
You also have to consider the Forbidden and Limited Cards list, which tells you how many copies of certain Cards you can play.