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
This directory must be deleted before installing mods.