IaaS provides raw infrastructure like VMs, networking and storage; you manage the OS, runtime and app. PaaS provides a managed platform/runtime where you deploy code and the provider handles OS, scaling and patches. SaaS is a complete application delivered to end users.
Expanding on the short answer — what usually matters in practice:
A tiny example (an explanation template):
// Example: discuss trade-offs for "iaas-vs-paas-vs-saas?"
function explain() {
// Start from the core idea:
// IaaS: Infrastructure (AWS EC2). PaaS: Platform (Google App Engine). SaaS: Software (Gmail,
}