A release in the curl project means packaging up all the source code that is in the master branch of the code repository, signing the package, tagging the point in time in the code repository, and then putting it up on the web site for the world to download.
It is one single source code archive for all platforms curl can run on. It is the one and only package for both curl and libcurl.
We never ship any curl or libcurl binaries from the project. All the packaged binaries that are provided with operating systems or on other download sites are done by gracious volunteers outside of the project.
As of a few years back, we make an effort to do our releases on an eight week cycle and unless some really serious and urgent problem shows up we stick to this schedule. We release on a Wednesday, and then again a Wednesday eight weeks later and so it continues. Non-stop.
For every release we tag the source code in the repository with "curl-release version" and we update the changelog.
We had done 172 curl releases by January 2018, and for all the ones made since late 1999 there are lots of release stats available in our curl release log.
Every single change to the source code is committed and pushed to the source code repository. This repository is hosted on github.com and is using git these days (but hasn't always been this way). When building curl off the repository, there are a few things you need to generate and setup that sometimes cause people some problems or just friction. To help with that, we provide daily snapshots.
The daily snapshots are generated daily (clever naming, right?) as if a release had been made at that point in time. It produces a package of all sources code and all files that are normally part of a release and puts it in a package and uploads it to a special place (https://curl.haxx.se/snapshots/) to allow interested people to get the very latest code to test, to experiment or whatever.
The snapshots are only kept for around 20 days until deleted.