There are two main types of friction: static and kinetic. Static friction is the amount of resistance to motion when an object is static or not moving. Kinetic friction is the resistance on a moving object.
Did you ever notice how hard is is to move a heavy object, but once you start it sliding, it seems to move easier? It acts like it is almost stuck to to floor until you get it moving.
Although you need to overcome the inertia of the heavy object, much of the effort needed is due to the fact that static friction is usually greater than kinetic friction. Static friction is caused by a combination of the resistance due to the roughness of the surface and the molecular attractions trying to hold the surfaces together.
Once an object is moving, the amount of friction is less than when it was standing still. Sliding friction is a form of kinetic friction. Most of the friction is cause by the roughness of the two surfaces