Scale sets provide a way to create multiple VMs (based on the same image) dynamically as demand grows and shrinks.

Availability sets are a way of grouping VMs to spread them across multiple fault domains and update domains to provide high availability. Availability sets are a way to ensure that VMs are spread across different physical servers, switches, power supplies, etc, so that a failure of these does not affect all VMs.

What options are there for making a VM accessible from the Internet (e.g. public IP, load-balancer, NAT, etc)?