sterilizers kill free floating bacteria, algae, and parasites. Not everyone uses them but some folks swear by them.
Reactors run different media. You can run carbon or gfo
in a reactor. Check out Bulk Reef Supply for a list if reactors. They have videos on them and what they do. I haven't jumped into reactors yet myself but they look to be my next purchase. You can run media in a canister in occasion or with something like ClearFX Pro without a reactor.
I've had good luck with ClearFx Pro.