Plasmon is a quasiparticle resulting from the quantization of plasma oscillations.
Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric field of the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it. In most metals, the plasma frequency is in the ultraviolet, making them shiny (reflective) in the visible range.
Surface plasmons are those plasmons that are confined to surfaces and that interact strongly with light resulting in a polariton
Read more at http://en.wikipedia.org/wiki/Surface_plasmon