Specification and also an implementation for packaging, managing dependencies, deploying and executing applications composed from pieces/modules. Java users can choose from OSGi, NetBeans Runtime Container or possibly mix them together.
A bit of math
Module system design needs to face some tricky (e.g. NP-Complete) issues. Especially complex is management of module dependencies as described at LibraryReExportIsNPComplete. This can have various solutions (like LibraryWithoutImplicitExportIsPolynomial), but it also shows why good API design skills are needed and why BackwardCompatibility is important.
The need for Common Ground
Do you wonder why certain people think Ruby is better than Java? Do you think it is due to duck-typing? Due to having closures? Wrong, it is because Ruby has Gems. Listen to following screencast to get convinced that Java needs such common ground too. See it with your own eyes in action.