Solid, liquid and gas are the three states of matter we are all familiar with. We can move between the states by adding or removing energy (e.g. heating/cooling). If we continue to add enough energy (for a plasma, we add electrical energy), gas molecules will become ionised (lose one or more electrons) and so carry a net positive charge. If enough molecules are ionised to effect the overall electrical characteristics of the gas the result is called a plasma. Plasmas are therefore, quite rightly, often referred to as the fourth state of matter.
A plasma contains positive ions, electrons, neutral gas atoms or molecules, UV light and also excited gas atoms and molecules, which can carry a large amount of internal energy (plasmas glow because light is emitted as these excited neutral particles relax to a lower energy state). All of these species can and do interact with any surface placed in contact with the plasma. By choosing the gas mixture, power and other conditions we can quite precisely tune, or specify, the effects of the plasma upon the surface.