Explanation
Configuration Management (CMDB) is a product that allows users to identify and track components of their infrastructure, such as servers, applications, databases, networks, and devices, and their relationships1. CMDB provides a single source of truth for IT assets and services, and supports IT service management processes1.
Discovery is a product that automatically scans the network and populates the CMDB with the discovered infrastructure components and their attributes2. Discovery uses probes, sensors, and patterns to identify and classify IT assets, and updates the CMDB with any changes2.
Service Mapping is a product that creates a top-down view of the infrastructure components that support a specific business service, such as ecommerce3. Service Mapping uses discovery data and application traffic analysis to map the dependencies and relationships between IT assets and services, and displays them in a graphical interface3.
References
Configuration Management Database (CMDB) - ServiceNow1
Discovery - ServiceNow2
Service Mapping - ServiceNow3

Explanation