Friction is a force that slows down objects and is generated when object when 2 surfaces rub against each other. Friction does not have much of an affect on levers since the surfaces don't really rub against each other. On the other hand friction would have an affect on the inclined plain because when you use it surfaces area always rubbing against each other.
Links
Intro
Load Arm and Effort Arm
Levers and Friction
Classes Of Levers
Human Levers
Wheels
Bibliography