It is certainly more complicated than this, but I'll try to give a short description.
Forge and Forge Mod Loader are mods that allow people to easily write mods that are compatible with each other. In the past they had nothing to do with bukkit and bukkit plugins.
But, it also used to be that you could have a server with forge and bukkit. You can't do that anymore.
So, people are just starting to write mods/plugins that do stuff like protection, ranks etc. But there isn't a good, working one yet.