Friction
Static friction: Prevents an object from starting to move. Usually greater than kinetic friction.
Kinetic (sliding) friction: Resists motion once the object is moving. Always acts opposite to the direction of motion.
Friction force: f = μN, where μ is the coefficient of friction (property of the two surfaces) and N is the normal force (perpendicular to surface).
Rolling friction is much less than sliding friction — this is why wheels were invented.
Lubrication (oil, grease) reduces friction by separating surfaces with a thin film, preventing direct metal-on-metal contact.