Windows Insider Program

Windows Insider Program is a software testing program by Microsoft for Windows 10 and Windows Server. The user can opt in the program to test out new features and new versions of the operating system that were previously only accessible to developers. The first build that was released from the Insider Program was 9841. In addition, more builds are also released in ISO and VHDX formats for other platforms such as HoloLens, as well as development kits.

First model
The first model consisted initially of two rings, and received some updates over the time since its launch with the program until its succession early June 2020:

Skip Ahead: This was a special option available for the Fast Ring that could let a subset of Insiders on it to receive the earliest builds from a future update. For instance, Skip Ahead was enabled in early February 2019 for Insiders to move to 20H1 builds, when the May 2019 Update was still being finalized on the standard ring. In May 2020 Update build 19018, however, it was discontinued, with Insiders joined to it migrated back to the Fast Ring and the option for joining ones disappearing. This was for everyone on it to receive the latest builds at the same time.

Fast Ring: This ring received builds from the newest Windows 10 cycle, but not as early as those from Skip Ahead. Due to their earliness, some rough edges and instabilities could be find, which in future builds would be fixed. On mid-December 2019, this ring was updated to receive only builds from specific development cycles that were not part of any future update.

Slow Ring: This ring received builds less frequently than the Fast Ring, which were part of a near-final feature update prior to reaching the Release Preview ring. These builds were more stable than the Fast Ring ones, though they could still contain some minor bugs.

Release Preview Ring: This ring was first announced on 10 February 2016, and it allowed users to receive builds from the next Windows 10 feature update a few weeks prior to its general rollout. Once the version was released, this ring would keep getting the same servicing updates from consumers as well as other preview ones.

Second model
This model was first announced on 15 June 2020 as the evolution from the previous one and was eventually rolled out as a server-side update to the program settings on 29 June. It notably includes a "channel" rebranding that is more consistent and familiar with other testing programs such as Office Insider:

Dev Channel: This channel replaces the Fast and Skip Ahead rings, and it receives the earliest builds from Windows 10 development cycles that are constantly moving forward, which include the latest features and improvements from Microsoft's active development branches. However, they are not tied to any specific release of the operating system, and due to their earliness, some instabilities and bugs will be present that will be actively fixed.

Beta Channel: This channel replaces the Slow Ring, and it receives builds from the next Windows 10 feature update, including additional fixes and some features previously available only on the Dev Channel, while keeping system stability, like the October 2020 Update, which received some features from builds of the Iron cycle. Some minor bugs might still be present, though.

Release Preview Channel: This channel replaces the ring of the same name, and it receives builds from the next Windows 10 update once it reaches the final phases of its development, some weeks prior to its general rollout to consumers. After this, the channel will receive traditional servicing builds, including preview ones, while the next version is finished.