Topology 2 is a "region aware" view of the topology, where the administrator can configure the network into regions and sub-regions.
In its default state (where no "regions" or "layouts" are defined), it should look and behave similarly to (eventually, the same as) the "classic" Topology view.
Note that this view is currently "experimental", although the longer term plan is to bring it up to par with the "classic" view, which it will eventually replace.
A more detailed description of the current state of Topology 2 can be found HERE. (Steven to provide link)
The ONOS model of the network includes Region objects which can be configured with a collection of Devices (switches) "belonging" to that region.