Emacs Completion
Good article here:
https://mint.westdri.ca/emacs/intro_completion
Completion in Emacs refers to the ability of Emacs to select one from several options in a list, in the minibuffer. You see it in action when you switch buffers for example, or open a file.
There is also in-buffer completion, but that’s a different topic. See Company-mode for example.
Completion in the minibuffer traditionally the original Emacs completion function, completing-read. The documentation isn’t specific about how this happens, but calling completing-read will call up the minibuffer with a prompt, after which you start typing and you start hitting TAB to get more complete options.
Then came Ido and Ido-vertical which allowed you to select your items vertically in the minibuffer.
Then came Helm, which was basically a completely different Emacs completion
system. It was incompatible with completing-read, and so a lot of the
functionality that needed completion, like buffer switching, needed to have
helm specific versions.
Ivy, Counsel, Swiper
Then came Ivy. Ivy looks kinda similar to Ido-vertical, but it offers fuzzy matching and seemingly is better written.
Ivy sometimes gets bundled up with other package like Counsel and Swiper and I often get these packages confused. I wrote a page about it.
Vertico
Vertico seems to be the most recent, most modern completion framework, and it is compatible with completing-read, so the standard functions all work.
What does it mean to be compatible with completing-read?
One of vertico’s big selling points is that existing functions which use completing-read, like switch-to-buffer, will get access to Vertico’s functionality.
I’m not sure how this works exactly. In a very general sense, calling completing-read “outsources” to the minibuffer for reading input, and in some fashion Vertico enhances the minibuffer with vertical selection and an ecosystem of “add ons”, like marginalia and orderless, that further enhance the experience.
Look at consult as well, for some extra tooling around this.