What is " companion proceedings" in a conference? “All Web Science track papers will be published in the companion proceedings of the WWW conference." (http://www.www2015.it/call-for-web-science-track/)
What does " companion proceedings" mean？ What are the differences of it with the main proceedings?
There's no major difference between "main" proceedings and "companion" proceedings. It just means that there will be a proceedings volume that collects the papers. Perhaps the only possible item might be that it's not a dedicated series—it might be a special journal issue that serves as a proceedings volume.