In general, a service template is a set of resource types.
The purpose of combining resource types in a service template is to sell hosting resources in a useful package. When you open a hosting provider's webstore page, the resources are usually sold in the form of plan. The plans include such resources as disk space, available bandwidth, number of email accounts, subdomains, and so on.
Have a look at the picture below. This is an ordinary webstore with several hosting plans, each with its own resource limits.
By including resource types into the service template, you prepare a commercial offer. The next step is to define the prices for using the resources, that is, to make a service plan.
Note: Sometimes, a service template may contain the activation parameter which is already specified in one of the resource types included in this service template. If the value you set for the activation parameter on the service template level differs from the value on the resource type level, the system will use the service template value.