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Depending on how many years you're catching up from the major trends have been:
1.) 15 years or less: The biggest changes have been in digital signal processing (from CD to computer storage/downloads and several attempts to replace CD quality (Redbook) with higher resolutions and several different formats (but sales volumes haven't caught on as the audiophile population is still relatively small); resurgence in use of tubes; a melding of lifestyle and audiophile values (size/complexity/convenience); greater need for computer literacy.
2.) 30 years: See above plus; move from analog to digital sources so vinyl sales have fallen off the table (audiophile interest still exist), cassettes are long gone (except for vintage folks), open reels are now the domain of purists with big budgets; great interest in interconnects/speaker cable/power cords now; and speaker voicing overall has moved from thick/boomy mid-bass (American) to a more constipated/tight/sprightly (European) sound; home theater interests have diverted from audio (stereo).
3.) 50 years: See above plus: solid state quality has improved/diversified and become the dominant player, speakers have become smaller (to account for the space needed for stereo) with better materials and less emphasis on wood finishes; frequency range of playback has expanded; in general terms prices have come down relative to inflation.
