Mods



Mods (short for modifications) are anything that changes Minecraft's game content from what it originally was.

Purpose
Most mods deez nuts to the game to alter gameplay.

Client-based
Client mods are modifications to your game files themselves. They are not custom clients, and they require modification of minecraft.jar.

As the Minecraft server software will ignore custom content from client side, most of the client mods which add new content to the game do not work in SMP unless a modification has also been installed on the server. Some authors of client mods have made server versions of their mods available, and others have not. A few of the Loader/API type client mods (e.g. ModLoader and AudioMod) and many of the functional client mods (e.g. Optifine, Rei's Minimap and TooManyItems) have effect in SMP straight away, without any modification on server side.

Mod list on Minecraft Forums

Server-based
Server mods are modifications to the official Minecraft server software. They 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.

Mod packs
Mod packs are collections of mods that have been put together and configured so that they will all work together. Mod packs are often centered around a general theme like tech, quests, or magic. Mod packs often have either custom launchers or installers that make installing and running the mod pack easy. Some of the more popular mod packs are /Feed The Beast/, Tekkit and Hexxit. There are of course also smaller mod packs, such as Crafting Dead and Blocklaunch, most of which using the special Minecraft Technic Launcher from Technic Pack.

Most mod packs can be found on custom launchers, which make it easy to install and launch various mod packs.The most popular of these are the Technic, Feed the Beast, and AT lauchers. In addition to making it easy to install mod pack clients, certain launchers can also download mod pack servers.

Bugs
If you install a mod, it may stop the game from working properly. If you encounter a problem while using mods and want to report it, first remove all of your mods and see if the problem still happens. If the problem continues to happen, it can be reported on the bug tracker. Otherwise, try to reproduce the problem with the fewest possible number of mods, and then report the problem to the author(s) of those mods.

If Minecraft crashes, a modified game is flagged in the crash report. The following link explains how to obtain a crash report. (Note: If your Minecraft has never crashed, following those instructions won't find anything.)

The crash report text will include a line near the bottom which will say one of:

Is Modded: Probably not. Jar signature remains and client brand is untouched Is Modded: Very likely; Jar signature invalidated Is Modded: Definitely: client brand changed to (present loader, such as "fml,forge" or modloader)

A shortened example crash report is given below:

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The code that checks for mods is fairly simple, and it's not always correct; it may say 'probably not' even with mods installed. However, it's very difficult to get the 'very likely' message if you haven't modified minecraft.jar somehow, so that's essentially a 'yes'. There's also a 'definitely' message, seen when a Bukkit server crashes and under other similar circumstances, like when the Minecraft Forge API is installed.

'Probably not' appears when the client/server brand appears to the in-game check to be unaltered (often termed 'vanilla') and the META-INF folder is still there. 'Very likely' appears when the META-INF folder is not present but the client/server brand seems to be vanilla. 'Definitely' plus the client name appears when the client is not vanilla. For example, you might see something like:

Is Modded: Definitely: Client brand changed to 'fml,forge'