What does a standard Yu-Gi-Oh deck consist of?

Answer 1

A standard Yu-Gi-Oh! deck has Monster, Spell and Trap cards.

1. The ratio of Monster cards to Spells + Trap cards is about even.
2. There are at least 40 cards. You can have more, but sticking close to 40 is better when it comes to your chances of drawing a particular card(s).
3. Fusion and Synchro Monsters (Purple and White card backgrounds respectively) are optional and are put in a separate pile called the "Extra Deck" face-down. There can be no more than 15 cards in the Extra Deck.
4. A mix of of lower level monsters and higher ones. Why? Monsters with 5 or more stars cannot be Summoned except by Tributing a monster already on the field. Only a few high level monsters should be used unless you use a strategy to get around Tributing or makes Tribute Summons easier to perform.

And that's basically it for a standard deck. Structure decks on sale have your basic cards as well as newer ones and have formidable strategies explained in a booklet that goes along with it.

As for more advanced decks, they are built on some of the same principals, but some decks employ strategies that make the above statements not appliable or not of concern for the deck. An example is a "Destiny Hero- Diamond Dude" deck with about 30 Normal Spells to make the most out of his effect.