Azure Reservations help customers to save money when they can make a duration commitment to an Azure service. Typically the duration is one or three years with, of course, the best savings to be made on a three-year commitment.
Today there are sixteen categories of Reservations ranging from Reserved Instances for virtual machines to Reserved Capacity for storage or database services, as well as Software Plans for Red Hat or SUSE Linux. Buying and managing Reservations, and understanding how discounts are applied can be confusing, but there’s some excellent Microsoft documentation available.
Find it here to dip into as you need to, or save it for the weekend and read it end-to-end: http://bit.ly/36W4KgJ.