Azure Reservations are a way of pre-paying for an Azure resource over a one or three-year term to get the most cost-effective pricing. The Reservations family continues to grow: it started with Reserved Instances, a way of pre-paying for Azure virtual machine base compute, and was extended with Reserved Capacity, a way of pre-paying for Azure SQL Database compute capacity. Today the family also includes Software Reservations for SUSE Linux software, and Reserved Capacity for Azure Cosmos DB throughput.

This page (http://bit.ly/2Q3vm7J) gives you an overview of Reservations and if you expand the “Buy a reservation” link at the left you can get details on the current four members of the Reservations family.