Wikidata is the wikipedia for knowledge, i.e. give everybody the possibility to freely share all knowledge.. It’s a bit a freeform tag-value pairs, but both sides are linked to other entries with the same thing, so they can be translated and related.
A problem with wikipedia at the moment is that even the English one has information of only a limited subset of places of the world (e.g. Africa is seriously underdeveloped). But in other languages that’s even worse. With Wikidata, the situation improves quite a lot – for instance Wikidata has more entries for places in Europe than the English Wikipedia (because it collects all the languages).
Wikidata is a quite successful wikimedia project when you look at the number of editors, but most importantly it has brought in more than 6600 new editors who hadn’t contributed to any wikimedia project yet.
Initially, Wikidata was just a technical proposal, with nothing to show. This gave high and conflicting expectations. Now that there is something visible, the expectations crystalise and converge.
Wikidata started with the adding of language links to Wikipedia. This brought in a community from the start, and created a lot of Wikidata contents quite easily.
Wikidata rolled out first on some smaller wikipedia, and only after that on the English one. This allowed to solve the technical issues before the big storm.
Wikidata makes it possible to store conflicting data. For instance, how many people live in Israel? Multiple answers are possible, Wikidata allows to store all of them.
Wikidata’s software doesn’t restrict what you enter in it. Some people want databases to restrict content, e.g. Gender = Male or Female. Wikidata intentionally doesn’t do that. This has an impact on the community, i.e. who is involved in the project and how they interact.
Wikidata consists of a data store (= wikidata.org), and a client (which runs on the wikipedias).
There is a library (DataModel) that can deserialize the data that comes out of wikidata, which can be used by plugins or other tools in PHP. There’s also a query library (Ask) – but the QueryEngine counterpart doesn’t exist yet.