A service is a name for the operation the user was doing. As such, services group together similar user interactions (for example, paying an invoice or adding an item to a shopping cart).
Secondary purpose of a service is determining the slow threshold to interactions consuming this service. Transactions exceeding this threshold would be flagged as slow. Out of the box the slow threshold is 5s, but it can be overridden for a any service from the service settings.
Service detection works differently depending on what type of application Plumbr is monitoring.