The Wikimedia Foundation is the organization that carries the responsibility of Wikipedia's ongoing health and success. Sadly, it seems that as the Wikimedia Foundation has grown, it has failed to develop a good understanding of the unique asset, i.e. the loose-knit "community" of volunteers who produce Wikipedia's content, that it has, or the conditions that will promote that community's health and productivity.
Below is my adaptation of a graph created by Marc, which roughly describes how various entities in the Wikimedia sphere interconnect. It may be somewhat useful, but I think it needs further improvement. Generally speaking, "Wikimedia" is an umbrella term, which refers as much to "the collection of websites that includes Wikipedia, Wikisource, Wikidata, etc." as it does to the foundation, so it needs to be a distinct entity. MediaWiki and WikiBase are software platforms, created and maintained by the Wikimedia Foundation, and serving the websites (which are known as "projects" in our jargon).
digraph { layout=dot rankdir=BT overlap=false concentrate=false bgcolor=lightblue // splines=" " "Wikimedia\nFoundation" -> Wikimedia Wikimedia -> { Wikipedia Wikidata Wikiversity "Wikimedia\nCommons" Wikibooks Wiktionary Wikisource } "Wikimedia\nFoundation" -> MediaWiki "Wikimedia\nFoundation" -> WikiBase MediaWiki -> Wikimedia MediaWiki -> WikiBase WikiBase -> Wikidata Wikidata ->{ Wikipedia Wikiversity "Wikimedia\nCommons" Wikibooks Wiktionary Wikisource } }
Another notable graph related to Wikimedia is the one (flawed) that emerged from Wikimedia's 2010 Strategic Planning process (which puts "technology" at the core), and the (in my view, corrected) version that puts "community" at the core. See this essay: What Wikimedia Needs in a Trustee 
(it was not titled with an eye to continual relevance, but the issues discussed there remain).