Learn more about servicenow discovery basics here

As per servicenow docs,ServiceNow Discovery finds applications and devices on your network,and then updates the CMDB with the information it finds.Discovery is available as a separate subscription from the rest of the Now Platform. See Activate Discovery for details.

Learn more about servicenow discovery basics here:

Discovery finds computers, servers, printers, a variety of IP-enabled devices, and the applications that run on them. It can then update the CIs in your CMDB with the data it collects.

https://docs.servicenow.com/bundle/london-it-operations-management/page/product/discovery/concept/c_GetStartedWithDiscovery.html

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Q&A from sevicenow community website

ServiceWatch:

ServiceWatch performs top down discovery of business services. In very simple terms, you give ServiceWatch an entry point (how users access the application/business service), which in most cases is an URL, and ServiceWatch maps out the application top down (from the entry point down to the db / storage layer). Once an application is mapped out you can integrate servicewatch with any of the supported monitoring connectors and you’ll have a living, “service aware” map of your application. As for your change control policy, ServiceWatch can definitely notify you of any unplanned changes to devices captured in the business services maps that are active.

ServiceWatch does not have a CMDB component on its own. Currently, ServiceWatch integrates with ServiceNow (Fuji) and any active business services are pushed through to ServiceNow and serviceWatch CI information can be viewed in ServiceNow. However, in the ServiceNow Geneva release (either this month or next) ServiceWatch will be integrated on to the ServiceNow platform and CI’s discovered in ServiceWatch will populate in the ServiceNow CMDB out of the box.

Discovery:

Your use case for discovery is the same reason why we looked into ServiceNow in the first place. Discovery / CMDB is definitely your answer to getting rid of the disparate / inconsistent keeping of data in excel files, sharepoint sites, etc. With discovery, you basically “boil the ocean” in terms of scanning your entire network and pulling back all information possible from desktop/laptops (pc and macs), servers (physical and virtual), databases, storage arrays, network devices, etc. in your network. You’ll have a single source of truth for all devices across your company.

Refer:

https://community.servicenow.com/community?id=community_question&sys_id=2c721f61db101fc01dcaf3231f961934