ModLoader was a type of mod manager and organizer created by Risugami (on the Minecraft Forums). It helps eliminate conflicts between mods: useful if you have many mods installed at the same time. Almost all mods required ModLoader in order to be used without problem. Coding to make mods work for ModLoader was a bit harder for beginners, which is why most newbie mods are not for ModLoader, although it is a very efficient program if you want to have mods. The ModLoader is NOT supported by Minecraft. 1.6.2 was the last version that was worked on and has now been replaced with Forge. The last version ModLoader was released for was 1.3.2. Many people still use it to make mods which are compatible with older versions. See the Mod Making Tutorials on how to use ModLoader to program and create your own mod.