In Azure is a logical grouping capability for isolating VM resources from each other when they’re deployed. Azure makes sure that the VMs you place within an Availability Set run across multiple physical servers, compute racks, storage units, and network switches. If a hardware or software failure happens, only a subset of your VMs are impacted and your overall solution stays operational. Availability Sets are essential for building reliable cloud solutions. They’re made of Fault Domain(s) and Update Domain(s).
The hardware in a location is divided into multiple update domains and fault domains. An update domain is a group of VMs and underlying physical hardware that can be rebooted at the same time. VMs in the same fault domain share common storage as well as a common power source and network switch.
SLA for an Availability Set is of 99.99%