Information comments can be both submitted and viewed by visitors on the information page. Depend on comment settings may a visitor can fill out a comment form and submit it for publication on the site.
Categories are containers for information. With categories you can organize information into a logical structure, so your visitors will able to find them easy.
The information is the essential part of GrandCMS. All information is a different page of your site. You can organize them under categories, you can search them, you can use different layouts for them and you can use them for many more. Actually the whole system is made for to show your visitors this information.