IaaS
In plain terms
Cloud model providing raw computing infrastructure.
Going deeper
Infrastructure-as-a-Service rents virtual machines, storage, and networking, giving you control over the OS and software while the provider runs the hardware. It's the most flexible cloud tier.
Under the hood
You manage OS upward; more control and responsibility than PaaS/SaaS.
Examples
EC2, Azure VMs, GCE, virtual servers, custom infrastructure.
Think of it like...
Like leasing empty land and utilities, then building whatever you want on it.