What is the difference between EFS and S3?

What is Amazon EC2? How is it different from traditional on-premises physical servers?

EC2 lets you rent a computer in the cloud instead of buying and maintaining your own physical server...which is higher on price...

What is EBS optimised instance,how is it related to instance pricing..need a real time example

what is the main difference between dedicated instances and dedicated host?

Is it possible to change the type of instance after launching...like on - demand to reserved instance?