Fabric

Fabric is a lightweight modloader. It has a separate API, required by most but not all Fabric mods in the mods folder. It has been worked on alongside the 1.14 snapshots of Minecraft Java Edition since before snapshot 18w49a (before December 5th 2018). It was created in 2016 as a hobby project and was publicly announced in 2018.

Fabric is version agnostic, which means it can be used on any version of Minecraft. There are even examples of it being used on other Java-based games. This is not applicable to the API of the same name, which is officially available for all Minecraft versions down to 1.14.4, various snapshots and the Combat Test versions of Minecraft.

Starting from around 20w46a build numbers for the Fabric Loader versions have been removed. Versions now appear as "0.10.6" instead of "0.10.6+build.214".

Although Fabric is less popular than Minecraft Forge, there are over 4000 mods for it.

Fabric mods can be commonly found at 3 sources: Curseforge, Planet Minecraft (though not as easy to filter) and Modrinth.

Fibermc is a useful website for discovering, searching and filtering Fabric mods that lead to Curseforge download pages and is planned to support Modrinth, Github and Glass Repo in the future.

There is also dedicated updated fan forum threads for Fabric starting from 1.14, 1.15 , 1.16 and 1.17 snapshot and release versions to track mod updates, present Mod Developer modpacks, host installation help and other useful information for users to view related to each version.

Fabric also has many projects that split off into different areas such as Legacy Fabric (Fabric on versions before 1.14), Cursed Fabric (Fabric on alpha and beta versions), Jumploader (Fabric support on the Curseforge launcher, this was before Curseforge's launcher added a Fabric Loader option around 1.17's release), Patchwork and Forgework API (Forge mod compatibility patched to work on Fabric then with Forgework the reverse) and more, these are accessible via another fan forum.

Background
Fabric, like Rift, was created in response to the allegedly slow development of Minecraft Forge around Minecraft version 1.13. The developers considered Forge to be bloated, so the Forge API functionalities were either split out into Fabric API or discarded. Fabric itself is only a modloader.

Fabric also maintains its own obfuscation mappings, released under the public domain CC0 license. Unlike Forge's MCP and the official Mojang mappings, there are no licensing restrictions placed on Fabric's mappings, which allows them to be used for any purpose. Automatic tools that match between versions are used, so any new snapshot or pre-release version quickly gets supported.

Trivia

 * Fabric is the second time a modloader has had a large support for 100s of mods consistently alongside Forge since Risugami's modloader. (Meddle last 1.9 to 1.11 snapshots with 18, Rift lasted 1.13-1.13.2 with around 55 for 1.13 and 43 for 1.13.2; with Liteloader only lasted from 1.5.2 to 1.12.2 with around 48 and many stopping at different versions).
 * Fabric is the second modloader to support snapshots (Meddle being the first supporting 1.9 to 1.11 snapshots).