Friction is a consequence of energy lost as heat whenever there is a relative motion between different components of a system. If a block of wood is pushed on a level ground the molecular bonds that cause adhesion at its interface with ground are constantly broken and recreated. This process generates heat.
At a molecular level, there is no friction. However when we consider macroscopic bodies, we ignore several degrees of freedom while considering macroscopic motion. Friction is due to the motion along these ignored degrees of freedom. (For example, while considering the motion of the block of wood, we assume that all particles of the block are moving in a single direction. While in reality, the molecules at the interface are being pushed around randomly. And of course, all molecules have a thermal vibration.)
Another example of friction could be a fluid in motion. Different layers of a fluid some times move at different speeds. The molecular adhesion leads to transfer of momentum across layers giving us the macroscopic concept of viscosity.