A configuration management database (CMDB) is the record of the configuration items (CIs) that deliver your IT services, and of how they depend on each other. In ITIL 4 it supports the service configuration management practice: knowing which servers, applications, network devices and cloud resources a service needs, so incidents are diagnosed faster and changes are assessed against real dependencies.
A CI is not the same as an asset. An asset is something you own and pay for, tracked for cost, contracts and lifecycle; a CI is anything you need to manage to deliver a service, tracked for its relationships. A laptop is usually an asset first; a database server behind your ERP is a CI first. Many records are both, which is why the CMDB and IT asset management in HaloITSM share the same data.
The CMDB is one of the ITIL practices described in our practical guide to ITSM. HaloITSM is the tool we recommend most often for it: see why HaloITSM is our pick.
Source: the ITIL 4 Service Configuration Management practice, ITIL 4 by PeopleCert.