Feather duster worms - colloquially, at least - are worms from the taxonomic order Sabellida (which, to my knowledge, currently includes the families, Sabellidae, Serpulidae, Fabriciidae, and Siboglinidae). Some people break it down with "fan worms"/"feather duster worms" being from Sabellidae, "tube worms" being from Serpulidae, and "giant tube worms" from Siboglinidae. However, many worms build tubes (so I find those common names not very descriptive/helpful), and all of the above families consist of worms that send out fans/feather duster like crowns to filter feed.
So, personally, I lump all worms from the order Sabellida into the category of "feather duster worms." (Though if I recognize the species and it has a more specific common name - such as "Christmas Tree Worm" or "Coco Worm," for example - then I'll use typically use that name to talk about the species instead of the more generic "Feather Duster Worm" name.)
