It is software that is used to publish blog websites!
Its very good software, open sourced thus cost effective and extremely flexible to do more than just blogs (web logs).
WordPress although best suited for blog websites, can be easily configure to be used for more traditional websites with many pages and a full menu structure, including a full shopping cart and many more really good plugins.
WordPress Content Managed websites have been around for a long time, so why blog about it?
To answer my own question, as a hosting company and web developers that primarily develops content managed websites (websites that can be edited easily via an admin or back-end facility) we have noticed a trend towards WordPress websites for a few reasons:
IDS WebDesign is focussing more on WordPress as a Content Managed Website solution and have the in-house skills to develop the required templates.