There are three types of Decks in Yu-Gi-Oh!

The Main Deck
It is essential for Dueling and consists of 40 to 60 Cards.
Monster Cards, Spell Cards, and Trap Cards can be placed here.

The Extra Deck
It is not essential for Dueling. Extra Deck Monsters like Synchro, XYZ, Fusion, and Link are placed here. It is limited to 15 Cards.

The Side Deck
It's also not essential for Dueling. Cards you want to keep handy while Dueling in order to adjust your Deck to your opponent go in here. It is limited to 15 Cards.