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 services provide virtual machines (compute), networking, and attached storage you manage (OS, patches, runtime). They differ mainly in pricing options, instance families, and ecosystem integrations.