Deep web: stuff not indexed by search engines. Private forums, non-public social media accounts, Telegram rooms, Discord servers etc. are technically "deep web".
Dark web: a subset of deep web that requires specific software or configuration to access. Slightly more precise, but still includes every possible use case for IPFS, Dat, ".onion" etc. Note that this is nowhere close to what people usually mean when they use the term "dark web". They're referring to the subset of a subset of deep web that's used for criminal activities.
The problem is that there is one (academic) definition of "deep web", but many incompatible definitions of "dark web", invented by the media basically for whatever they want it to be.
