One major difference between N64 emulator plugins and plugins for PC software is that the former are essential components of the system, whereas the latter tend to be optional add-ons. This frees up authors to work on whichever parts of an emulator they enjoy or are good at etc. Zilmar' is someone who's been a great asset the emulation community for many years. He also wrote a simple set of example plugins to demonstrate the system to would-be authors. The specification is not set it stone, there have been several updates already.
Currently definitions exist for three types of plugin - video, audio and input.
Video plugin, as you might guess, handle all the graphics output of an emulator and are thus extremely complex projects in themselves, perhaps of comparable difficulty to writing a core. Audio plugins, in case you haven't guessed, handles all music and sound effects, and are also all HLE, and are also very hard things to write. Emulator authors are free to choose which of the three parts of the spec. Extract the .dll file from the archive you downloaded, into this folder. Consult emulator documentation for more information on this.