What is the difference between PaaS and app server hosting?

In terms of deploying a java web app on the internet, what is the difference between sites that offer app server hosting such as Tomcat, Jboss and Glassfish, and sites that offer platform as a service such as Google app engine, 开发者_开发知识库Heroku and Amazon EC2?

Difficult question, since PaaS is often misused term. But I'll try: PaaS usually involves (another) layer of abstraction (you usually don't really know on which server exactly your app is running). Plus your application may or may not share the hardware with one or many other tenants.

Traditionally providers of app server hosting just used N (like 1000) customers per server and calculated that way. GAE etc. try to optimize (distribute the load more cleverly) this and usually charge you on what you actually use.

IMHO: In theory you could use more resources on a app server than you actually pay. But since they usually over-sell, you'll find it hard to get what you pay for. In theory, with pay-what-you-use PaaS offerings you get should what you pay for. /IMHO