Client.jar

Not to be confused with the launcher marked for "Linux / Other", minecraft.jar is the main code source for Minecraft. It is located in the ../.minecraft/versions/(current version) directory, or the ../minecraft/versions/(current version) directory in OS X. It can be opened using a file archiving program.

Jar versions
The Minecraft .jar file is found in its version folder. It is accompanied by a .json file that lists the version's attributes. Normal versions are available to choose from via the launcher, and snapshots are also available. It's even possible to download alpha versions, and create your own version file to install mods on. Note that creating a new version is the only way to install mods; the launcher keeps normal versions synced.

Files in minecraft.jar
In minecraft.jar many files can be found, they can be categorized like this:
 * Texture files: Textures used by Minecraft, all in .png format.
 * Class files: Compiled Java code that Minecraft uses for program logic. These files are named like "acq" or "qn" because they have been run through an obfuscation tool to hide the names of things. However, they can still be disassembled to understand their functionality.
 * Language files: Language files are used to change the messages 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: Contains information about the game files, rather than information explicitly required by Java to run the program.

minecraft.jar hierarchy
As of Minecraft 1.7.10. Java class files have been omitted for brevity. Sorted by type, then by name.

Minecraft.jar Minecraft.jar Minecraft.jar Minecraft.jar Minecraft.jar