You might think of a DITA map as the Swiss Army Knife of lists. When you do a search on the web, you get back a list of topics that match that query. When you sort business cards from a conference by company or job title, you are creating a hierarchical list of people by that category. Similarly, a DITA map is basically a list of resources that fits a particular reading sequence and hierarchy, possibly one of many depending on how the information was organized.
Did you know?
Imagine the branches of topicrefs in a map as being either numbered lists of links (items that should be read in a particular sequence) or as bulleted lists of links (simply lists of resources that your reader can select from at will). In the topicref at the start of a group of links, the attribute collection-type='sequence' will generate Previous/Next links just as if your reader were following a numbered set of links in a table of contents. If instead you change that value to collection-type='unordered', the generated links in each corresponding topic will simply be a list with no reading order implied.