Flutter is oscillations in various modes and combinations of modes induced by mass distribution, elasticity and aerodynamic properties of an aircraft and its parts. Aeroelasticity is the theory of how that all is related to each other and how to predict how an aircraft design would behave. Typically flutter appear and increases as airspeed and dynamic pressure increases.
Rod Machado - Aerodynamic Flutter (5 min)
FlightInformer - The flutter effect explained (8 min)
NASA Langley CRGIS - Aeroelastic Phenomena and Related Research - Part 1 and 2 (6 and 7 min)
Jeff Quitney - Flutter (Aerodynamics) circa 1940 NACA Langley Research Center (15 min, no audio)
To round it off, here is probably a bit of a classic:
NASA Langley CRGIS - Aeroelasticity Matters (5 min, with some music)