Data templates describe the data which Cacti is going to store in the RRD files. This basically comes down to the RRDtool create command. Having a template for the RRDtool create command issued internally by Cacti assures you that RRD files based on this template are always created in a common way.

Please note that once a data template is being used to create an RRD file, changes to the data template will not be reflected on that RRD file.

Graph templates define the look and feel of a graph. They also provide the skeleton to the RRDtool graph function, defining the data sources to use and the graph items to display. Changes to a graph template get propagated to all graphs based on that template.

Host templates are like shopping baskets for graph templates and data queries. Let's assume a specific device type should contain several different graphs. Instead of adding each single graph template and data query to each single device, you can simply define these within a host template and assign that host template to the device.

Unfortunately, changes to a host template do not get propagated down to the single devices.