Wikimedia is the name of a movement, and also part of the name of several organizations. The Wikimedia movement wants to support content under the concept of free and open content. Wikipedia is the most famous product of the Wikimedia movement.
The movement consists of:
- The Wikimedia Foundation, which is the owner of the Wikimedia wikis such as Wikipedia and Wikidata
- several other organizations and groups that are allowed to use the Wikimedia trademark, such as the national affiliations (also called chapters)
- many volunteers who are active on the wikis or in the organizations.
Note that Wikimedia, Wikipedia, Wikidata etc. are trademarks owned by the Wikimedia Foundation. 'Wiki' on the other hand is a generic term for a certain kind of website. This means that not all websites or organizations with the name 'wiki' in it are related to the Wikimedia movement. (For example, 'Wikileaks' has nothing to do with it.)
In order to avoid confusion, it is best practices to use the specific name of the organization you talk about: Wikimedia Foundation, Wikimedia Deutschland, etc. The different organizations have different roles, tasks and responsibilities in the Wikimedia movement.
Wikimedia Foundation (WMF)
The Wikimedia Foundation is based in San Francisco, USA. It has the final decision on the wikis. In general, the people on the wikis (the communities) are more or less free what to do and what to decide together for their wikis.
The Wikimedia Foundation is a non profit organization. It is funded mainly by money donated by hundred of thousands of readers who love Wikipedia. Sometimes the WMF is supported by a special grant to work on a specific project.
Jimmy Wales, by the way, is the founder of Wikipedia and of the Wikimedia Foundation. He is one out of 10 members of the board of the WMF. He is the owner or co-owner of several enterprises which are not related to the Wikimedia Foundation.
Other Wikimedia organizations
The national (and other, like thematic) Wikimedia organizations are independent organizations, but they work together with the Wikimedia Foundation and the volunteers. Usually, such an organization is a members association. You are welcom to become a member, even if you are not active on a wiki. These organizations have the task to support the volunteers in order to provide more and better free and open content. These organizations have no say about the wikis; for example, Wikimedia Italia cannot decide what has to happen on Wikipedia in Italian.
Most organizations are happy to inform about Wikidata. They may not always have the possibility to answer in detail, or host an event especially dedicated to Wikidata. But for most questions they are a good starting point.
One of the national affiliations (chapters) has a special role with regard to Wikidata: the chapter for Germany, Wikimedia Deutschland. It hires most of the developpers (software engineers) who create and evolve the software for Wikidata.
Although the Wikimedia Foundation and some chapters have staff, nearly all work on the wikis is done by volunteers. For example, volunteers on the wikis write Wikipedia articles, upload pictures to Wikimedia Commons, or correct data on Wikidata. Content creation is volunteers work. Some content comes from external sources. Those 'content' donations must be free and open content, as nearly everything in the Wikimedia movement.
Some Wikimedia volunteers are not active on wikis. They help to make the organizations better, for example as board member with experiences from outside the movement. Other volunteers are good in explaining the concept of free and open content and do outreach projects. So the Wikimedia movement is much more than 'writing Wikipedia articles', it needs the talents of many different people.