Friction is the resistive force that occurs when two surfaces interact, opposing the relative motion between them. It acts parallel to the surfaces in contact and can be either static or kinetic.
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Friction depends on the nature of the surfaces in contact and the normal force pressing them together.
Static friction prevents objects from starting to move and is generally greater than kinetic friction, which opposes ongoing motion.
The coefficient of friction ($\mu$) is a dimensionless quantity representing the ratio of frictional force to normal force; it varies for static ($\mu_s$) and kinetic ($\mu_k$) friction.
The formula for calculating frictional force is $f = \mu N$, where $N$ is the normal force.
Friction converts kinetic energy into thermal energy, leading to wear and heat generation at contact points.