An instance group is a collection of many instances that share the same configuration. Developers use instance groups to set uniform policies and rules across multiple instances easily. All instances in an instance group go through the same lifecycle simultaneously.
Single instance vs. multi-instance
Single instance describes a cloud infrastructure setup where only one instance of the compute resources is created and allocated to the user. Meanwhile, multi-instance is a term used for more than one identical instance that run parallel to each other. In a multi-instance setup, each instance has its own compute resources.
Single tenant vs. multi-tenant
Single tenant means the cloud infrastructure is exclusive to one user. This is common on an on-premises server, where the organization might run multiple workloads on a single machine. Meanwhile, a multi-tenant setup shares the cloud infrastructure amongst multiple users. This allows them to scale workloads on the cloud at a lower cost.