What is a modded server?Edit
A modded server is a normal server, but running either a modified jar, such as Minecraft Forge, or MCPC+; MCPC+ is generally favoured, as it allows a combination of mods and plugins.
Every player who connects to a modded server will need exactly the same version of forge, exactly the same mods, and exactly the same config files. This greatly increases the complexity, so it is recommended you either provide a download link to all the correct mod versions, or create a modpack that can be used from the Technic Launcher (other launchers are available).
Modded servers use significantly more memory than their Vanilla counterparts. It may be necessary to upgrade your server to serve the same number of players.
Unfortunately, bugs are far more liable to occur with mods; from corrupted chunks, to crashes due to faulty mods. Most are fixable, but you may need to keep a closer eye on your server.