What's the difference between dash and dock?

I've seen a lot of talk about the Dash to Dock extension, but I'm really struggling to see the difference between dash and dock.

As someone who also has extensive experience of Mac, it seems the OSX dock has almost exactly the same functionality as GNOME dash.

What am I missing?

It is terminology. In default GNOME Shell, the "Dash" is the bar with favorites and running applications that appears in the Activities Overview. It is only visible in the Activities Overview, which you see when you click "Activities" in the top bar, or hit and release the Super key.

A GNOME Shell extension, "Dash to Dock" can turn that Dash into what from then on is referred to as a Dock on your desktop. Then, it is directly available on your desktop, and thus behaves much more like a dock in for example MacOS.

In a default Ubuntu install, a custom GNOME Shell extension, Ubuntu Dock, is active. That is actually derived from "Dash to Dock" and uses the same configuration settings. So you already have the "Dock" instead of the "Dash", and do not need to install an additional extension for that.

I'm as uncertain as you, but this seems like an opportunity for clarification.

This I know is the dock:

Then there is this group of screens:

• Activities Overview
• Activities Overview (in searching mode)
• Apps Overview / Apps Grid

I originally assumed (perhaps inaccurately, as revealed later), they together are making up what's called dash.

I arrived to this conclusion partly through my memories from using Unity. In Unity it was more clear, because it occupied only the top left part of the screen, and had this distinct border, and of course it needed a name, and got named dash. And in it, you performed the same tasks as you now do in these screens.

I also tried to verify this theory by doing an image search for "Unity dash" and "Gnome dash"; the results didn't seem to contradict.

Activities Overview screenshot:

Enter the rabbit hole

The unexpected tale of the gnome shell extension Dash to dock:

What we need to establish first, is that Ubuntu is not responsible for much of the Gnome UI. Gnome is a third party "software foundry", whose software (the part responsible for the UI, gnome-shell) gets used by Ubuntu.

But of course, Ubuntu needs to carry out some customizations to gnome-shell before they can present the entire release to us. Even Ubuntu is using gnome shell extensions to achieve some of this (as opposed to trying to maintain an Ubuntu-favoured gnome-shell fork).

Now, thanks to Vanadium's answer, cursory findings from my earlier, briefly relevant experimenting, and the annotated illustration in the referenced gnome-shell Wikipedia post, I have put it together:

In the original unmodified gnome-shell (without the Ubuntu distro's customizations), the part we in Ubuntu know as Dock,

• is only a "subcomponent" of the Activities Overview (and Apps Grid) screen, available only together with it, while you are in the activities overview mode. If you leave the Activities Overview, this subcomponent disappears too, and can't be accessed otherwise.
  • as you can observe in the above screenshot of the Activities Overview, the vertical bar with the app launcher icons on the left side is present
• in original gnome-shell terminology however, this subcomponent is called — wait for it — Dash.

The gnome shell extension Dash to dock (shipped with and enabled in Ubuntu by default) "extracts" this subcomponent (at that point, still called Dash) out of the Activities Overview, sticks it to the edge of your desktop permanently, and invites you to refer to it as Dock from then on.