It may seem obvious, but unless stated otherwise, a model does not have a special rule. Most special rules are given to a model by the relevant entry in its Warhammer army book. In addition, a model's attacks can gain special rules because of the equipment it is using. Similarly a model might get special rules as the result of a spell that has been cast upon it or perhaps even as the result of it being in a particular type of terrain. Where this is the case, the rule that governs the equipment, spell or terrain feature in question will make this clear. Unless otherwise noted, the effects of different special rules are cumulative. However, unless otherwise stated, a model gains no additional benefit from having the same special rule multiple times.
Most of the more commonly used special rules in Warhammer are listed here, but this is by no means an exhaustive list. Many troop types have their own unique abilities laid out in their Warhammer army book. Remember, that if there is a conflict between the rules presented here and in a Warhammer army book, the latter takes precedence.