Client.jar

Minecraft.jar is a main system file for Minecraft, it is located in the ../.minecraft/bin directory, or the ../minecraft/bin directory in OS X. The minecraft.jar can be opened using an archive opening program (e.g. WinZip, WinRAR, 7-Zip, etc.).

Files in Minecraft.jar
In the Minecraft.jar many files can be found, they can be categorized like this:
 * Sprite files: Files that contain sprites (2D textures) used by Minecraft, all in .png format.
 * Class files: Compiled Java bytecode that Minecraft uses for program logic.
 * Language files: Language files are used to change the text in the game (achievements, tooltips, menu). It does not affect in-game chat.
 * Text files: Files that contain easy to edit text, such as splashes and texture pack descriptions, another text file is achievements/Map.txt that contain code for the creation of the achievements menu background.
 * The META-INF directory

The META-INF directory
META-INF stands for meta information about the program. It contains information about the game files, rather than information explicitly required by Java to run the program.

This directory is notable because it must be deleted to use mods. The META-INF folder contains a digital certificate (MOJANG_C.SF and MOJANG_C.DSA) that contains a checksum of all the Java files. The certificate is issued to prove the code is safe, and is what lets the web applet version of Minecraft run without Java safety dialogs. When the files are modified, they could have been tampered with, and the certificate is invalid (the checksum changes). If a .jar file contains an invalid certificate, it won't run. Removing META_INF removes the certificate, so it will run again, but if you try to use the browser version of Minecraft, you might get warning dialogs. The MANIFEST.MF file is unrelated, and simply contains information about the archive.

Deleting the META-INF directory has no important side effects but could theoretically open the door for "viruses" in the form of poorly or maliciously written plugins. Be careful when modding your game. See the Mods page for details.

Removing this folder in order to successfully install mods is no longer required as of Beta 1.9pre.

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