You type something in a form and hit "post this"; in the next second it's on your website. Your posts are then automatically arranged by date and category into a skin/theme that you can chose (and customize if you wish to do so).
Readers of your website can then leave comments which will appear below your post. They can also browse through the archives and use advanced search capabilities.
b2evolution also generates web pages dynamically from the MySQL database, so no clumsy 'rebuilding' is involved. It also means faster search/display capabilities, and the ability to serve your posts/news in different 'templates' or 'skins' without any hassle.