Cloud services come in three layers. IaaS (Infrastructure as a Service) rents raw computing power, like servers and storage. AWS, Azure, and Google Cloud together hold roughly two-thirds of this market.

PaaS (Platform as a Service) adds development tools on top, so teams can build apps without managing the hardware. Heroku and Google App Engine handle that part for them.

SaaS (Software as a Service) is what most people actually use. Gmail, Slack, and Netflix are all software running on someone else's servers, reached straight from your browser. The model removes upfront costs and turns computing into a pay-as-you-go utility.