A package diagram in the Unified Modeling Language depicts the dependencies between the packages that make up a model.
Packages containing nodes and artifacts.
In addition to the standard UML Dependency relationship, there are two special types of dependencies defined between packages:
A package import is "a relationship between an importing namespace and a package, indicating that the importing namespace adds the names of the members of the package to its own namespace."  By default, an unlabeled dependency between two packages is interpreted as a package import relationship.
Package diagrams can use packages containing use cases to illustrate the functionality of a software system.
Package diagrams can use packages that represent the different layers of a software system to illustrate the layered architecture of a software system. The dependencies between these packages can be adorned with labels / stereotypes to indicate the communication mechanism between the layers.