Plone 4.2 has introduced a new Collection content type, which replaces the Topic content type in previous versions of Plone 4.
With the older Topic content type, users could create a collection (or canned query) of related content based on a number of criteria, such as content type, published status, creation or modification dates, author, and so on. While this was convenient, it was also somewhat cumbersome to add each of the criteria to the collection. The Topic first had to be created and saved, which would then allow the user to access a Criteria tab where they could specify their parameters. Also, criteria had to be added one at a time, without any feedback as to what content was being matched. This lead to an error prone process that required the user to jump back and forth between the editing and viewing mode to see if their criteria were correct.
With the new Collection type, the criteria are defined as search terms. These search terms served the same purpose as criteria, but are much easier to add, since the initial creation page allows users to add terms right away (see screenshot below).
In addition, the new creation process allows users to see a live preview of the content that matches their criteria, therefore making it possible to see what existing content will match the saved criteria. Internally, the new Collection type is relying on a new query system that will unify content queries throughout Plone.
And for users who want to have access to the older style Topic collection, the two can live side-by-side.
More information is available in the Plone 4.1 to 4.2 upgrade notes.