Platform as a Service is a cloud computing model in which a third-party provider delivers hardware and sfotware tools -- usually those needed for application development -- to users over the internet. A PaaS provider hosts the hardware and software on its own infrastructure.
PaaS vs SaaS vs IaaS
Platform as a Service
Software as a Service
Infrastructure as a Service
Heroku
Heroku is a commercial service provider. You can create yourself an user account, and get a great service and hosting experiance for your applications and code.