“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.
Often a "main" proceedings collects the papers from the main conference tracks and "companion" proceedings collects the papers of satellite events, such as workshops and such things.
In some cases, only the "main" proceedings is considered as a publication (in Computer Science, at least) and the "companion" proceedings is not.