Jack (Java* Android* Compiler Kit) is a new Google* tool that includes a compiler from Java source code to the Android dex file format. Jack has its own .jack library and provides most toolchain features as part of a single tool: repackaging, shrinking, obfuscation and multidex. There is also a tool that translates existing .jar files to the .jack library format. This is Jill (Jack Intermediate Library Linker).
Overview - When the tool is enabled, Jill will translate any libraries you are referencing to a new Jack library file (.jack). This prepares them to be quickly merged with other .jack files. The Jack and Android Gradle plugin collects any .jack library files along with your source code and compiles them into a set of dex files. During this process Jack also handles any requested code minification (shrinking and/or obfuscation). The output is then assembled into an apk-file as normal.