Copying all or part of Wikipedia, and developing it independently of the original, is forking. Typically, a fork is implemented as another website (though printed forks are possible). There are forks of the whole encyclopedia, and there are specialized encyclopedias that start as a copy of a collection of articles from Wikipedia about a specific subject. Wikipedia's content is made available in data dumps. A list of forks is available at Wikipedia:Mirrors and forks/All.
What is the difference between a mirror and a fork?
A mirror is an exact copy. A fork is a copy that has been changed, diverging from the original path of development, like a fork in the road. Mirrors provide alternative access to Wikipedia (when access is needed offline, or when the Wikipedia site is down). Forks allow you to start with Wikipedia content, and transform it into something else.
Wikipedia considers each Wikipedia article to be an individual document. Moreover, for the purposes of creating derivative works of individual Wikipedia articles, Wikipedia considers a direct link-back to a particular Wikipedia article as being in full compliance with the Creative Commons Attribution-Sharealike 4.0 Unported License (CC BY-SA), provided your work is also licensed under CC BY-SA.
How much storage is required for a copy of the English Wikipedia?
The data dump of all pages and their histories is a several-gigabyte file, which according to the warnings about decompression size at the database download page, could theoretically decompress to at least a terabyte.
How do I install Wikipedia's software?
Wikipedia runs on MediaWiki. Make sure to check if your computer meets the minimum system requirements, install it on the official website of MediaWiki, and follow the instructions.