What's the difference between a Catalog and a Desktop Group in Citrix XenDesktop 5.5?

Citrix XenDesktop 5.5 aggregates Catalogs, Desktop Groups, and Active Directory (AD) security groups together to dictate which users get what virtual machines (VMs). You should already know what a security group is. A XenDesktop Catalog identifies a collection of VMs that can be managed as a single entity. A Catalog can represent VMs atop a specific hypervisor or with a specific class of storage. A Catalog of VMs can then be subdivided into one or more Desktop Groups, and each of those is then assigned to an AD security groups. Desktop Groups can be either Shared or Private. They can also be constructed as Application Desktop Groups, which are used for hosting XenApp-published applications. 

Source: http://www.windowsitpro.com/article/virtualization2/catalogs-desktop-groups-xendesktop-142245