From the ServiceNow Wiki on Discovery
The ServiceNow Discovery application finds computers and other devices connected to an enterprise's network. When Discovery finds a computer or device, it explores the device's configuration, provisioning, and current status and updates the CMDB accordingly.
On computer systems, Discovery also identifies the software that is running and any TCP connections between computer systems. Discovery creates all the relationships between computer systems (such as an application on one server that uses a database on another server).