Cloud computing is the delivery of computing resources (servers, storage, databases, networking) over the internet on a pay-per-use basis. Instead of buying and maintaining your own hardware, you rent what you need from a provider.
Service models
IaaS (Infrastructure as a Service) — rent VMs, disks, networking. You manage the OS and middleware. AWS EC2, Google Compute Engine
PaaS (Platform as a Service) — the provider manages infrastructure; you deploy code. Heroku, Google App Engine, Render
SaaS (Software as a Service) — a finished product accessed via a browser. Gmail, Slack, Salesforce