The value of a ward save will always be shown in a model's entry in the relevant Warhammer Armies book. These work in the same way as armour saves, and may be combined with other Ward saves as normal. The key difference between ward saves and armour saves is that ward saves are never modified by the S of the attack. However, no model may have a Ward save better than 4+ by combining multiple Ward saves regardless of source.
Note that does not stop single Ward saves from being used, such as a model having a listed 2+ or 3+ Ward save; the above limitation only applies to combining Ward saves.
Sometimes a model has both an armour save and a Ward save. Where this is the case, the model takes its armour save as normal. If the armour save is failed (or modified to the point at which the model cannot pass it) then the model takes its Ward save.
Some models may be allowed to re-roll a Ward save from a specific source. In that case, the re-roll will only apply to that specific Ward save, and not the model's total Ward save.