AWS On-Demand Instances (Amazon Web Services On-Demand Instances) are virtual servers that run in AWS Elastic Compute Cloud (EC2) or AWS Relational Database Service (RDS) and are purchased at a fixed rate per hour. AWS recommends using On-Demand Instances for applications with short-term, irregular workloads that cannot be interrupted. They are also suitable for use during testing and development of applications on EC2.
AWS EC2 instances are available in a variety of different levels of compute power and are designed for different tasks within the cloud. On-Demand Instances have no contract commitment and can be launched as needed -- except potentially during periods of very high compute demand in particular availability zones. This is generally the most expensive purchasing option for AWS instances, though exact hourly prices vary depending on the operating system and size of the instance. Each On-Demand Instance is billed per instance hour from time it is launched until it is terminated or stopped. Partial instance hours are rounded up to the full hour upon billing. A free tier of usage is also available.
AWS On-Demand Instances can be launched using the AWS Management Console or Amazon's RunInstances API. Users can launch up to 20 On-Demand Instances at one time; a request form is required for launching more than 20 instances. Some individual instance types are further limited in number or by availability zone.