Minecraft wiki entry for Client.jar, associated text: Class files: compiled Java code that Minecraft uses for program logic. These files (classes) have names such as "acq" or "qn" because they have been run through an obfuscation tool to hide the names of methods, variables, and classes. However, they can still be disassembled to understand their functionality.
The META-INF directory: contains meta information about the game files.
Resource pack files: Contents that are in the vanilla resource pack, located in the assets directory.