Friction

"Friction is the dissipation of kinetic energy into internal energy in the form of agitation of atoms throughout the objects.  We say that friction is a "dissipative" process."

Friction can either speed up an object or slow it down, depending on its motion.  For instance, on an airport conveyor belt your bags initially have no momentum, but with the addition of the movement of the conveyor belt and friction forces opposing the bag staying in its zero position, the bag begins to move in the direction of the conveyor belt.  We can model this sliding friction with the equation:$$F_{KF} = \mu_k F_N$$$F_N$ is the "normal force," or what is causing the objects to contact each other.
$\mu_k$ is the coefficient of kinetic friction, a value that will range depending on the material.
The speed of an object does not effect the sliding friction force.

 If a force is applied to an object that does not exceed the friction force, $F_{friction}$, we say that you are being counteracted by the static friction.  In order for movement to occur, the force applied to an object must be greater than or equal to the frictional force in the direction of the application.$$F_{SF} <= \mu_s F_N$$$\mu_s$ is the coefficient of static friction.  $\mu_s$ will more than likely be larger than $\mu_k$ because once an object is already moving, it requires only as much force as it already has to continue its movement.  You can feel this effect when you manage to slide a heavy object across the floor, and although it is difficult initially it becomes much easier once it is moving.

Any force of at least $F_{SF}$ in value can be applied to an object, but once it is exceeded, the equations governing its frictional status changes to sliding, $F_{KF}$.