Client.jar

Minecraft.jar is a main system file for Minecraft, it is located in the "../.Minecraft/bin" directory. 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: code files that minecraft uses.
 * 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: the title/splashes.txt is the least of splashes, and the achievement/map.txt is a code used to generate the achievement screen's background
 * The META-INF directory

The META-INF directory
META-INF stands for meta information about the program. rather than information explicitly required by the program.

This directory is notable because it must be deleted before installing mods. The META-INF folder contains information about the files that should be used in run-time, therefore alerting your computer to ignore mods and "safely" crashing if the mods edit the original class files. Deleting this folder blinds Java to these safety procedures and allows it to run the modded code.

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 you game. See the Mods page for details.