ModLoader for Minecraft can handle most mods but there are some that it can't which is where ModLoaderMP for Minecraft comes in.
ModLoaderMP works with mods that ModLoader doesn’t such as Flans Plane Mod, SDK Guns Mod and Minecraft forge. ModLoaderMP is mainly for Mods that can be used on Minecraft Multiplayer Servers, hence the name ModLoaderMP (Multi player). The installation process for ModLoaderMP is exactly the same as ModLoader for Minecraft but you need to put it in the server files. To install it, simply delete the META INF file to prevent a black screen, drag the mod loader files into the .jar and then install the corresponding mod as per the individual instructions.
ModLoaderMP for Minecraft allows you to run almost any multiplayer Minecraft Mod.