Containers are best known as the wrappers or decorative features around information that is displayed on a website. They are similar to skins in the design application and can be applied to a module, a page, a site and with some clever coding, appear on only certain areas of a website, without having to do anything, eg.. it's managed within the skin.
They are great for isolating content, creating a 'visual standout' and highlighting feature information.
Containers can make an impact on a page, by providing images instead of text for headings, or extra padding between content, or helping break up a site and providing whitespace between content areas.
XD Design doesn't use many containers in this design. It just looks like it on some pages, and with a little bit of patience, understanding how modules work, you can create some cool looks with or without containers.