Those are 3 different things. Let’s review.
That’s the portion of the World Wide Web that is freely, directly accessible, and searchable with standard web search engines. Obviously you know that one and use it every day.
It might come as a surprise for some of you, but surely you do use it every day too.
It’s basically the opposite of the surface web. Also called invisible web or hidden web, the deep web is all the parts of the World Wide Web that are not indexed by standard search engines. That’s the definition provided by Wikipedia, but I believe a site tagged “noindex” is still surface web, as long as access is free and doesn’t require registration or some specific technology (cf darknets below).
For instance, most parts of Facebook (all content that isn’t posted as “public”) is part of the deep web. Your web mail, your online banking accounts, contents behind a paywall, etc are “deep web” too. Told you you use it every day 😉
Before talking about the dark web, we need to talk about dark nets (with an s).
Wikipedia provides a pretty good definition there: “a darknet (or dark net) is an overlay network that can only be accessed with specific software, configurations, or authorization, often using non-standard communications protocols and ports.”
Nothing to add, really. For instance, Tor is a darknet, Freenet is a darknet, GNUnet is a darknet, I2P is a darknet, etc. So, again, there are several darknets. Let’s say, about a dozen “big” ones.
Wikipedia is straight to the point again: “The dark web is the World Wide Web content that exists on darknets” (so you see why I had to define a darknet first).
So whatever is on Tor, Freenet, etc, is on the dark web (and also on a dark net).
It is typically said that the dark web forms a small part of the deep web. I prefer to consider them as 2 distinct entities, as the barriers are distinct enough: the deep web is behind a login wall, the dark web is behind a networking technology “wall”. Also, some parts of the dark web (like some marketplaces) are behind both a networking technology and a login wall.
But if you are comfortable enough with those notions to start arguing about this detail, you know enough already 😉 (and sadly, you also know more than the average IT journalist :/ )
Bonus: clearnet, opennet, etc
Clearnet is a term used by dark web users to describe the regular internet. Since most darknets are focused on anonymity, it’s a convenient way to talk about “that part of internet where you are tracked”. Contrarily to what used to be mentioned on Wikipedia’s “surface web” page, clearnet is NOT synonymous with “surface web”. Facebook is clearnet, but as we saw earlier a lot (most?) of it is deep web.
Opennet is a more uncommon term. I only know about it in the context of Freenet. In the Freenet network, you can use 3 ways to connect: you can either connect only to people you know (real life friends), or connect to strangers, or do both. When you connect only to people you know, Freenet calls that “darknet”. When you connect to strangers, Freenet calls that “opennet”. Even though technically both are “dark nets” in the usual definition, at Freenet’s level the difference makes sense.