Akonadi is the storage management platform for the KDE Personal Information Management suite (KDEPIM). It's actually a pretty good software package, if you're into that sort of thing.

Nepomuk is semantic desktop engine for KDE, which tracks metadata across the filesystem. The 'semantic' element refers to tags and ratings, but it also tracks ordinary file properties (such as location, date modified) and their contents. Strigi is the crawler which scans the filesystem to record those in the database. Virtuoso is that database. Soprano relates to the storage format used by that data, IIRC.

I find they perform well, and speed up file searches, so I keep them enabled. Generally, Strigi will try to scan your system while you're not using it. If it's slowing your system down by scanning it during use, you can right click on the Nepomuk icon in the system tray to disable scanning. You can also set limits for the Nepomuk database size in System Settings, so that it uses less storage on the filesystem and less memory.

I would recommend keeping it, but it's easy to disable completely if you never use it.