Collections are essentially indexes of Mura CMS content items based on pre-defined search criteria and/or filters.
For example, Mura CMS can create an index of content items which have been flagged as "Featured" and categorized as "Domestic News" where the content resides under either the Blog or News sections of your site. You can control the maximum number of items you wish to display and more.
You can also choose to broadcast Collections as RSS feeds (http://en.wikipedia.org/wiki/RSS), making it easy for visitors, or even other web sites, to consume information from your site and bring visitors back for details.
In addition to creating Collections of Mura content items, Mura CMS can also consume external RSS feeds and display pieces of that information on your site using the Collection display object, and "Remote Feed" as the Content Source.