Services and service groups
Services are the ones that gather data from a host.
Once you have configured a host, you need to add some monitoring items to start getting actual data.
A service is an individual metric. One way of quickly adding many items is to attach one of the predefined templates to a host. For optimized system performance though, you may need to fine-tune the templates to have only as many services and as frequent monitoring as is really necessary.
In an individual service you specify what sort of data will be gathered from the host.