Minecraft Server Mods essentially offer a means of modifying or enhancing-the-features-of a minecraft server. They are generally developed (and supported, in most cases) by third parties (not Mojang AB).
Mods can make the game more exciting or bring in more variety, more possibilities and add more features – basically, to enhance the fun in the game. They can also help in managing administrative tasks, better.
Management mods – These mods are developed to make the administrative tasks easier. They generally do not alter the .jar minecraft server file. They are usually implemented as wrappers (can be thought of as actual wrappers which do not enter or modify the minecraft server, but wrap around it), and might monitor inputs to and outputs from the server. For example, mods for automating back-ups or protecting the server from griefers, etc.
Examples:- McAdmin, SimpleServer Wrapper, AsiWrapper
Gameplay mods – These mods involve modifications to the game mechanics. Therefore there most probably will be changes to the .jar server file. Bug fixing mods and new feature addition mods, belong to this category.
Examples:- Bukkit, Canary Mod, LeafCraft SMP
Some mod makers, provide an API (Application Programming Interface) or functions, that can be used by developers, to make their own plug-ins/installers, for offering a new feature or making a modification to the game. Bukkit is an example. The Bukkit software to be installed on the server is referred to as Craftbukkit. This bundles the Bukkit API with the minecraft server. The developers use the API functions to make plug-ins, which can be installed on the server. Thus, Craftbukkit (API) forms an interface between the minecraft server and the plug-ins.
Some time back, hMod developed by Hey0 was a very popular mod used in minecraft, which offered various functionality. However, this mod is no longer updated anymore and Hey0 himself has suggested that users use Bukkit. Unlike Bukkit, which is implemented as an interface with plug-ins, hMod included all functionality in itself (no plug-ins). It had thus become bloated and cluttered. Bukkit is very similar to hMod in terms of the functionality provided, but differs in the way of implementation. It has been designed to keep mods and the server more clean and easy to manage.
Check Bukkit Minecraft Server Mod for more details.
Since mods are made by third parties, be wary of downloading and installing them. Do so only if the mod-provider is trustworthy. Here again, finding out whether a mod-provider is trustworthy or not, can be tricky. There has been an instance in the past, where a popular mod-maker played with the trust of his mod’s users. The mod had a secret back-door, wherein the mod-maker could go to any server running his mod, and make admin-level changes, to the extent that the actual admin of a server, can get banned from his own server.
The mods mentioned in this article are just for your information. minecraftserverhosting.org takes no responsibility for any damage/unwanted-modifications caused on the installation of any of these mods.