Friction can be increased by increasing the surface roughness of the objects that are in contact while friction can be decreased by smoothing out the objects or stopping the objects if moving. There are two primary types of friction used in science: static friction and kinetic friction.
Static friction is friction that happens when two objects are not moving relative to one another. An example of this kind of friction would be a desk on the ground. Kinetic friction is friction that happens when two objects are moving relative to one another. An example of this kind of friction would be a sled on the ground.
In science, friction is often quoted by two coefficients. A coefficient for static friction and a coefficient for kinetic friction. This is because the frictional force is assumed to be proportional to the coefficient of friction. Most of the time, however, the amount of force that it takes to move an object from its starting resting position into a moving position is greater than the force that it takes to keep the object moving once it has already started moving. For this reason, scientists will use the two coefficients. Friction is a complex phenomenon that cannot be represented by a simple model.