A Package is a namespace as well as an element that can be contained in other Package's namespaces. A Package can own or merge with other Packages, and its elements can be imported into a Package's namespace. In addition to using Packages in the Browser window to organize your project contents, you can drag the Packages onto a diagram workspace (most diagram types, both standard and extended) for structural or relational depictions, including Package imports or merges.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, pp.241-242) states:
A Package is a namespace for its members, which comprise those elements associated via packagedElement (which are said to be owned or contained), and those imported. A Package definition can extend the contents of other Packages through the merging of the contained elements.
A Package may be defined as a template and bound to other templates: see sub clause 7.3, Templates, for further information. The URI can be specified to provide a unique identifier for a Package. Within UML there is no predetermined usage for this, with the exception of profiles (...). It may, for example, be used by model management facilities for model identification.