Infrastructure as a Service (IaaS) refers to a service that would traditionally be a server solution you deploy in your own data center (on-premises), for instance storage and virtual machines.
An IaaS service allows you to easily add capacity and the hardware is maintained by the cloud provider. The costs and features of different clouds are largely similar when it comes to IaaS capacity. Some differences do exist, but they may not be very relevant for choosing a cloud provider. The pricing is affected by the size of the server instances, the capacity type (on-demand or reserved), the operating time (start-stop), the architecture, the use of containers etc.
Overall, with IaaS businesses avoid investing in expensive on-site resources up-front.
Examples of IaaS are Amazon EC2, Azure Virtual Machines and Google Compute Engine.