Glowkit is a fork of the legendary Bukkit API, as continued by Spigot and Paper, intended for use with Glowstone.
Plugins designed for the original Bukkit API will continue to run on Glowstone, with zero or minimal changes.
The Glowkit Javadocs are available here.
If you'd like to take advantage of APIs not available in Bukkit, check out the APIs built into Glowstone itself.
Instructions on how to build Glowkit can be found on the wiki.