Friction is the force that resists relative motion between two bodies in contact.
Static friction occurs when the two objects are not moving relative to each other (like a desk on the ground). The coefficient of static friction is typically denoted as μs. The initial force required to get an object moving is often dominated by static friction.
Rolling friction occurs when two objects move relative to each other and one “rolls” on the other (like a car’s wheels on the ground). This is classified under static friction because the patch of the tire in contact with the ground, at any point while the tire spins, is stationary relative to the ground.
Kinetic friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction.
Sliding friction occurs when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction.
Fluid friction is the friction between a solid object as it moves through a liquid or gas medium. The drag of air on an airplane, or that of water on a swimmer, are two examples of fluid friction.
Friction is important to keep us walking, driving, etc. Friction may also “bad” since it can cause damage to devices, and is a reason for wear. It is saif that “half of all energy produced is spent to overcome some type of friction.”