Paper is a high-performance fork of Spigot, designed to optimize Minecraft server performance and add additional features.
While Spigot is a widely-used server platform based on the Bukkit API, Paper offers enhanced performance through optimizations that reduce lag and improve server efficiency, especially for large player bases.
It also provides extra features not found in Spigot, such as better world generation controls, more detailed configuration options, and additional API enhancements for plugin developers.
See also: https://www.spigotmc.org/wiki/what-is-spigot-craftbukkit-bukkit-vanilla-forg/