A Plugin is a piece of code that you can link into another program. An example would be you have a custom 3d geometry file format for your 3d engine, yet you are using 3d studio max to generate your art. So you can either write a tool that you run on the 3ds files that outputs our special format or you can write a Plugin specifically for 3d studio max that allows you to save to your special format from within 3d studio max eliminating the need for a stand alone tool.

A Filter is usually a process that changes an image or frame. You may have a gamma correction filter that goes through your textures and corrects the colors for you target hardware ( For instance on the N64, strong reds get way over saturated, so to eliminate this you reduce the red portion of all your textures by a set amount. This process could be considered filtering. ) Another example would be a filter that warps your final image after youve rendered everything to your framebuffer, much like Ready To Rumbles knockout screen warping effect or Devil May Crys fade screen/feedback effects.