The NagVis code is separated into several layers. One of these layers is the "data layer". The data layer is used to handle requests for information from third party sources like the socket of MKLivestatus. We call this layer "backend" in general. There are some components inside NagVis to manage these backends which act as the glue between the single backends and the other layers.
At the moment there are a few backends delivered with the NagVis core packages: the most important ones are the mklivestatus and ndomy backends. All backend configuration parameters are described in detail on their own pages.
The ndomy backend has been the default backend from NagVis 1.0 to NagVis 1.5. It fetches Nagios information from the NDO MySQL database. Since NagVis 1.5 the default backend has been switched to MKLivestatus backend.
In NagVis 1.9, the pgsql backend has been added to fetch the Nagios information from a PostgreSQL database as used by e.g. Icinga 2.
The backends are defined in the main configuration file. See main configuration format description on how to define backends.