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.

It currently supports 1.14.4 and 1.15.2. It also supports the latest snapshot, 20w21a, & combat test (currently combat test 5).

Background
Fabric 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.