Bristle worms are polychaete worms which are a annelid worms just like the oligochaeta , a group that includes the common earth worms.
They do sort of look like an earth worm with bristles.
Bristle Worm.
Some people absolutely hate having them in a reef aquarium because the bristles can poke into the human skin causing a lot of pain and they are not easy to remove.
If that ever happens to you try some duck tape (I am serious).
On the other hand they are useful detrivores that eat decaying organic matter such as food left over by other tank inhabitants, or dead inhabitants. They can be opportunistic predators and actually attack other animals.
Some grow quite large (and start to look like a snake) and can even bite you at that point they can become a real pest in a reef aquarium, but some people have kept even really large specimens for years without a problem.
Large Bristle Worms (about 1cm wide).
I so far like them and try to get as many as possible.
See what other people think about bristle worms:
What To Do With Bristle Worms? — The Reef Tank