Pixel Gun Nexus is a mod of the game Pixel Gun 3D - specifically its version 11.3.1 - that aims to enhance gameplay experience and in general revive the old version it is based on.
Pixel Gun Nexus also adds more content to the game, such as content from newer versions of the game or new, custom made content such as weapons, maps and features in general.
Pixel Gun Nexus has a ton of additional content and features, both in-game and in the servers. Here's dumbed-down and short list - which doesn't include everything - of key things that have been added: