Amazon Elastic IP address is assigned with our AWS account (5 Elastic IP per account), not to a particular Amazon EC2 instance. This number can be increased by requesting to the AWS team. Please give up the additional unused Elastic IPs to AWS else your account will be charged.
It is an Internet routable IP address that can be attached to an Amazon EC2 instance. The same Elastic IP can be detached and can be remapped to another Amazon EC2 instance. It takes 30 seconds to few minutes to reflect with the EC2 instance.
When an Elastic IP is associated to an EC2 instance, the instance's existing Public IP address mapping is removed and is no longer valid for this instance. A new Public IP will be reassigned to this Amazon EC2 instance when we de associate the Elastic IP from this instance.
There is no notable performance difference when we are contacting an EC2 instance using Elastic IP or private IPs.
Elastic IP will come handy when we are designing EC2 failover mechanisms in AWS. Example: We can have 2 MySQL DB servers running in Master-Master mode in the AWS. When one of the EC2 Master fails we can detect it using Linux HA heart beat algorithm and remap the Elastic IP of the original master to the secondary master. Now the Secondary master starts serving the request from the Application servers using the same Elastic IP.