Microsoft Sentinel is a Security information and event management (SIEM). A SIEM system combines outputs from multiple sources and uses alarm filtering techniques to distinguish malicious activity from false alarms.
A lot of security solutions exist which cater to specific needs of an organiation. A SIEM sits at the centre itegrating and collecting data from all these solutions.
Defender for Cloud was originally designed to be a Cloud Workload Posture Protection(CWPP) service which can help protect all infrastructure in Azure and other major clouds.
Defender for Cloud also includes Azure Defender which is the EDR/XDR solution for Cloud and selected on-prem workloads. More details can be found here.
MIcrosoft Sentinel is Microsoft's cloud native SIEM (Security Information and Event Management) + SOAR(Security Ochestration and Response) and has the capability to monitor and remediate several workloads in Azure, Other Clouds as well as On-Premises.