You can think of the Opus as an on-line birding encyclopaedia that everyone can edit. Each bird, location, piece of equipment etc has a 'page' in the article, and this page can contain photos, sound files and references (links) to other pages - for example a page about Skomer in Wales will probably contain references to birds of particular interest that you can see there.
When you edit the page, the past version is kept as a 'history', and anyone can clearly see exactly what changes have been made. This can be seen on the recent changes page, linked to in the left hand navigation on every page, and by clicking the 'history' tab of a page. This means that even if someone, for some reason, decides to delete the contents of a whole page, we can easily revert it with a couple of clicks of the mouse.
How are the pages organised?
Each page can 'belong' to multiple categories - not just one. For example a bird may belong to the main 'Birds' category, but also 'Birds of Britain and Europe', 'Endangered'. This page, for example, belongs to the 'help' category and the 'beginners guide' category. You can click the category name at the bottom to view other pages that belong to the same category.
It is the responsibility of the Opus Editors to ensure that the proper categories are being used, and to set up the category structures in the first place.