Your Side Deck is a collection of between 1 and 15 Cards, separate from your Main Deck. The Side Deck is used only between Duels, to alter the contents of your Main Deck or Extra Deck to better suit a strategy against an opponent's Deck.
For example, a Burn Deck, a deck based around dealing direct damage through Card Effects and Spell Cards, could be easily countered by including several cards to prevent Life Point damage, such as Des Wombat.
However, a Side Deck is not compulsory, in which case, a value of 0 for a Side Deck is also considered legal. In both the OCG and the TCG, any value between 0 and 15 inclusive is permissible.
Rules for KDE Sanctioned Tournaments have not changed, however. For KDE Tournaments, you still must have either 0 or 15 cards exactly in your Side Deck.
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
ATK • DEF • Level • Attribute • Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua • Beast • Beast-Warrior • Dinosaur