Overview
Developer Mode is sort of 'cheating' on Halo: CE. It enables someone to easily kill AI opponents in SP maps or it allows a map designer to test out a map they just created.
Alldev
Alldev is devmode that works for Halo: CE multiplayer. It allows hosts to basically cheat. Non-hosts can use the lesser cheats, but cheats such as invincibility can only be achieved by the host.
In order to get Alldev, you have to download it.