Friction: The force that slows objects down when they are rubbing against each other. Some things make lots of friction, like brakes on a tire, and some make very little friction, like skates on ice.
Gravity: A force of attraction that occurs between all objects. We experience it as the constant force that pulls objects toward the center of the Earth.
Drag: The slowing down of something as it moves through air, water, or other fluid, which causes friction. A race car experiences drag because of air pushing against it as it goes around the track.