A passive effect is an effect that is applied as a part of a larger action, or is present throughout the game. They grant added bonuses, some of which directly increase base stats, and may require activation or a condition to be met.
Acquisition
There are currently three different ways to acquire passive effects in the game. The first is from champion abilities. Each innate ability has a passive effect, but some champions have an additional ability (or abilities) that grants a boost passively (ex. 
Sona's 
Crescendo).
The second source is from items. When purchased, these items will grant the wearer increased stats and effects while owned. These passive effects may change as through the build path, but usually remain consistent (ex. 
Zeal).
The third source is from global map effects. These include the Crystal Scar (Dominion) Proving Grounds (ARAM) the Twisted Treeline altar buffs, and the URF Mode buff. This passive effect applies to all players from the start to the end of the game.
Types
There are several different types of passive effects, aside from the usual defensive, offensive or utility. These mainly are:
Constant passives
These effects are persistent throughout the game no matter what action is taken. They include innate abilities, and both Crystal Scar and Proving Grounds passive bonuses.
Restrained passives
These are in effect until a certain action is taken that temporarily disables them.
Stacking passives
These passives have an effect which increases each time a certain action is performed.
Aura passives
These passives have a small area of effect that affect allies and or foes.
Buffing passives
These passives increase stats such as On-hit effect.
De-buffing passives
These passives reduce stats such as attack speed.
