Heroku is a cloud platform as a service. That means you do not have to worry about infrastructure; you just focus on your application.
In addition to what Jonny said, there are a few features of Heroku:
- Instant Deployment with Git push – build of your application is performed by Heroku using your build scripts
- Plenty of Add-on resources (applications, databases etc.)
- Processes scaling – independent scaling for each component of your app without affecting functionality and performance
- Isolation – each process (aka dyno) is completely isolated from each other
- Full Logging and Visibility – easy access to all logging output from every component of your app and each process (dyno)
Heroku provides very well written tutorial which allows you to start in minutes. Also they provide first 750 computation hours free of charge which means you can have one processes (aka Dyno) at no cost. Also performance is very good e.g. simple web application written in node.js can handle around 60 – 70 requests per second.
Heroku competitors are:
- OpenShift by Red Hat
- Windows Azure
- Amazon Web Services
- Google App Engine
- VMware
- HP Cloud Services
- Force.com