This guide, or rather a documentation, is my attempt to help make sense of those files that you may find in your Opera profile or preferences folder.
The information that I have gathered and provided below has no connection to Opera Software ASA and is the work of many bored hours spent online by yours truly. 
To find out about a file or a folder simply click on it to open a page with more details.
…/opera/
- cache
- dictionaries
- icons
- images
- mail
- opcache
- sessions
- skin
- styles
- temporary_downloads
- thumbnails
- toolbar
- vps
- widgets
autoupdate_response.xml- bookmarks.adr
- browser.js
- cookies4.dat
- download.dat
- fontswitch.ini
- global_history.dat
- link_queue_myopera.dat
- link_queue_out_myopera.dat
- lock
- notes.adr
- opcacrt6.dat
- opcert6.dat
- opera6.ini
- operaprefs.ini
- opicacrt6.dat
- oprand.dat
- opssl6.dat
- opthumb.dat
- optrb.dat
- optrust.dat
- opuntrust.dat
- override_downloaded.ini
- pluginpath.ini
- search.ini
- speeddial.ini
- tasks.xml
- typed_history.xml
- upgrade.log
- urlfilter.ini
- vlink4.dat
- wand.dat
- widgets.adr
