Wikibase Repository

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Functionality to create and manage a knowledge base, including user-defined properties
Mechanisms to access the knowledge base from other wikis and to update them on changes
A rich JavaScript-based user interface to easily access and update your data
A data model that takes knowledge diversity and multilingual usage seriously
Exports of the data in a number of formats like JSON, RDF/XML, N3, and YAML

On top of that, MediaWiki adds:

World-class internationalization and localization into more than 200 languages
Full histories of all previous versions of the knowledge base
A web-based interface with a number of features for wiki gardening, user security and identity, etc.
A platform known to be scalable for millions of readers and tens of thousands of editors

And extensions:

PropertySuggester - Provides suggested Properties when a user tries to add new Statements to an Item.
Documentation | Source (github)
WikibaseQuality - Three extensions to aid with data quality and checks.
Documentation | Core (github) | Constraints (github)

Wikibase Client

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Query and display data from a repository server on a client instance
Retrieve and display data using its Lua interface and parser functions
Immediately show changes of data from the connected repository

And extensions:

ArticlePlaceholder - Provides a special page with information about a certain item inviting users to create an article.
Documentation | Source (github)

Query Service

The Query Service is based on BlazeGraph and allows you to query the contents of a Wikibase installation using SPARQL.