Amazon EC2 is a web service that enables you to launch and manage server instances in Amazon's data centers using APIs or available tools and utilities. You can use Amazon EC2 server instances at any time, for as long as you need, and for any legal purpose. If you need 100 instances for a two-day research project, sure. If you need a fleet of instances that can be scaled up and down to meet the traffic fluctuations of your Facebook application, no problem.
Instances are available in different sizes and configurations. This allows us to provide different instance types that you can use to meet specific needs. For example, you might want to use an m1.small instance (one Amazon EC2 Compute Unit) as a web server, an m1.xlarge instance (eight Amazon EC2 Compute Units) as a database server, or an extra large High-CPU instance (twenty Amazon EC2 Compute Units) for processor intensive applications.
What makes Amazon EC2 different is that you use only the capacity that you need. This eliminates your need to make large and expensive hardware purchases, reduces the need to forecast traffic, and enables you to immediately deal with changes in requirements or spikes in popularity related to your application or service.