A wiki is a certain kind of website. There is no sharp definition of a wiki. Usually a wiki is a website that can be edited by 'everyone', by all visitors of the website. But there are different kinds of wikis: some are installed only for a small group of people for internal use. Also, not every wiki makes use of the concept of free and open content.
Wikidata as a wiki
Wikidata is a wiki. You can create and change pages. Such a change is called an 'edit'. Of course, there are rules you should know and follow. Other people on Wikidata can change or revert your changes. If there is a disagreement between you and another editor (user) of Wikidata, you should use a talk page to resolve conflict.
The core and most important thing of Wikidata (and other wikis) is its database. It contains all the content. The wiki pages are only one way to access the content. Especially when you deal with large amount of data, you may want to learn about other ways.
Wikidata and other Wikimedia wikis
The Wikimedia movement is a movement for (essentially) educational content in the broadest sense. The movement works with a lot of different wikis, such as Wikipedia and Wikidata. Of course, there are also other wikis from other movements or organisations.
Wikidata is a wiki for a general purpose database. But it also has a special task for the Wikimedia movement: the items can be linked to specific pages of the Wikimedia wikis. For example, a Wikipedia article often exists in several languages. Each language version of Wikipedia is a wiki of its own. Wikidata is the central place for links from language version to language version. At the moment (May 2017), Wikidata serves in this way only to Wikimedia wikis.