Mods

Mods or modifications are changes, additions, or deletions to the original game that alter gameplay in a way that was not originally intended. Mods are usually known as physical add-ons or changes that have to be installed into the original game. Mods that add new content to the original game are often called partial conversions, while mods that create an entirely new game are called total conversions, and mods that fix bugs are called unofficial patches. Mods for Minecraft can introduce an entirely new experience and increase the creativity in so many ways, and the Mojang team plans to add an official modding API that allows modders to more easily add content to the game. Most mods in Minecraft are add-ons that add content to the game which increase gameplay creativity or what a player can do when changing in the Minecraft world; while some may be more of a bigger "expansion", such as the Aether Mod, that add a significant amount of new content to the game. Other mods take away content to create a simpler or more specific gameplay experience. Some mods, such as the Portal Mod, attempt to bring over another game's content, experience, and feel. And still other mods do more technical things such as adding or expanding more settings/options or optimizing the speed, graphics, or gameplay of the game. Server mods or plugins mainly give server admins more options and ease of use, and most mods for single-player have a server version that allows or optimizes the mod in muliplayer. Mods for Minecraft are constantly being updated as the game updates - bringing more content, bug fixes, or optimizations. Many in the Minecraft community appreciate the additional experience and ease of use that come with mods, while others play "vanilla", the original game, with only the original content enjoying the game without the urges of making the game easier to play or the desire for cheating that can come when one starts installing mods. While mods for Minecraft are generally considered safe to install, one should still exercise caution with mods to prevent crashes, deletion of game or save data, system instability, or potential malware infections from a bad link or the mod itself. There are many mods or plugins available to complement the original Minecraft game or give server admins more options and control over their servers.

Harmful Mods and Mod Safety
Harmful modifications in Minecraft might contain viruses, or can corrupt save files. Potentially harmful mods usually are ones that add things like new blocks or alter their appearance. MCedit is usually quite safe, and TooManyItems mod, as they do not change any textures on blocks or add new content, thus no changes to any files or codes are made in the source. Some modifications that were designed for versions lower than that of the current version can be very harmful to your computer or copy of Minecraft, because they are not designed for that version. e.g. If you downloaded a Minecraft Mod from a website that was designed for version 1.6, and the current version was 1.7, the modification would not be compatible, and if run on a save file would most likely corrupt the file. But, if a mod was designed for 1.7, and it was 1.7.3, the mod could work, and it could also work on 1.7.4, 1.7.5 and so on, but you shouldn't rely on that, and try to find current versions of mods, compatible with the exact current version.

Mods that only alter the behavior of existing items, blocks, or mobs are fairly safe; they should typically be able to be installed or uninstalled at will without making a world save corrupted. Note that this includes mods that add crafting recipes without adding new items or blocks.

Mods that add content, such as blocks, items or mobs, are riskier to uninstall. Since 1.4, modded content is automatically removed from the world if the mods that added them are not installed, thus preventing crashes. This can still cause issues and uncertain behavior, even such as crashing.

Multiple mods may conflict with each other; certainly if two mods need to alter the same files, then they are unlikely to be compatible.

Client Mods
Client Mods are modifications to your Minecraft program (Alpha or Beta). They are not custom clients, and require modification of minecraft.jar. Like high resolution textures, they only work with the launchers, and won't work if you play Beta through a browser.

The most common form of client mods are aesthetic and/or functional. However, some client mods add content to the game, and others fix issues with the game. While some do work on SMP, mods that add content, and thus  could be used for griefing, do not work, as the server will ignore  custom content.

New Content
Any mods that add new blocks or items will have new Data Values, which can be found on the Mods/Data values page.

Introduction
You have to delete the META-INF folder inside the file "minecraft.jar" and replace the files "en_US.lang" and "stats_US.lang" inside the folder "lang" in order to use a language pack.

You can find a complete installation guide for different operating systems on the Minecraft Forums.

Server Mods
Server Mods are modifications to the official Minecraft server software. They generally fall into two broad categories: management and gameplay.

Management mods are commonly designed to make administration of servers easier by implementing tiered privileges for commands (such as kicking, banning etc.). They are frequently implemented as "wrappers" which do not actually modify the main server .jar file, instead monitoring its output and sending commands to it.

Gameplay mods are modifications to actual game mechanics, such as restricting the placement of blocks commonly abused by Griefers (e.g. lava, TNT), fixing bugs in the server (e.g. making furnaces function before the update of 10th of September 2010) or adding new gameplay features.

Mod List
This is a list of wrappers and modifications.

Beta Server Wrappers
There is a full list of server wrappers at the Server Wrapper page

Server Operating Systems
Server OS'  are often Linux distributions specifically built to host a Minecraft server.

They are built to ease the burden for Minecraft server administrators by providing tools to manage deployment & maintenance.