Infrastructure as a Service - renting basic computing resources like servers and storage from cloud providers. Like leasing an empty warehouse where you bring your own equipment and setup.
Amazon EC2 is IaaS where you rent virtual servers and configure them however you need, just like having your own computer in the cloud.
All four are core IaaS compute services that let you rent virtual machines (CPU/RAM), attach storage, and configure networking. You manage the OS, patches, runtime, and apps; the provider manages the physical hardware and data center.