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 of asiekierka and became publicly available in 2018.

Fabric is version agnostic, which means it can be theoretically 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 1.14.4, 1.15.2, 1.16.3, 1.16.4, various snapshots and the Combat Test versions of Minecraft.

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.