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Cauldron is a combination of Minecraft Forge and the Spigot project which to allow you to run a Forge server with plugins installed. This project was previously called MCPC+, and has since had a change of name.
Cauldron servers are very tricky to configure as the server version itself is a bit buggy, but becomes more complex when you are trying to balance mods and plugins so that they work. This is not a simple implementation and can make your server unstable, but it is the only way to combine both mods and plugins into a single server.
Keep in mind that all players wishing to connect will need to use forge with a matching set of mods to that found on the server. This makes it a big difficult for players to connect and could be an issue for growing the server.