EIPs are dynamically re-mappable static public IP addresses that make it easier to manage EC2 instances. Each EIP can be re-assigned to a different EC2 instance when needed. You control the EIP address until you choose to explicitly release it. An Elastic IP is associated with your account and not a particular EC2 instance. Since, public IP addresses are a scarce resource you are limited to 5. If you need more EIPs then you have to apply for your limit to be raised. If you have a large deployment then an elastic load balancer (ELB part of AWS) is placed in front of all the instances thereby consuming one EIP only.
You will be charged for all EIPs not associated with running EC2 instances. It is charged ...
