Controller concerns deal with routing events between the view and the model, keeping these two in sync, while taking care that the model and its presentation stay decoupled (Model_view_controller). Controller concerns are thus core to the specification of the dynamic behaviour of a UI.
The client-server nature of a web-based UI is a more restrictive execution environment than what is available to a normal GUI application. Hence, controller concerns in web applications typically boil down to deciding how to relay events generated in the browser to the model, and how to generate a web page (presentation) in response. Page-flow-centric approaches (see below) expand the scope of controller concerns in an important way to include the relationship between different pages in a UI.
Here is the taxonomy for strategies for dealing with controller concerns. See ControllerTaxonomyDiscussion for a discussion about the taxonomy.