SE/QA/Prod/Mobile - No piece of software can see such broad use by millions without constantly evolving to meet the ever-changing needs of users, the ever-changing blend of user agent / browser software, and the ever-changing and hostile environment of the internet itself. If you think you've ever seen a complex internet-based software project simply become perfect and then need no further changes for a decade or more, you clearly don't understand software engineering.

Discovery - This is about discover-ability of the content itself, e.g. search engines both internal and external, and other related matters.

Research - Try branching out a bit from the links and information at https://www.mediawiki.org/wiki/Wikimedia_Research/Research_a... for a more informed opinion

Security - Actually, a lot of people care about hacking an encyclopedia. Just on the server side they care about hacking the ~1000 servers that are servicing or operating on the private data of millions of users.

Traffic - It's a sub-set of Operations that deals specifically with the edge of the foundation's network (e.g. CDN-like things, for which they don't outsource a commercial CDN mostly for privacy reasons: spreading edge caches around the world, low-level performance optimization, SSL encryption, etc).

Cloud Services - This is where the foundation hosts virtual server resources for community volunteers to experiment with and run projects and products of their own that are relevant, e.g. "bot" software that patrols articles for likely vandalism attempts and such.