ELB is an AWS service that automatically distributes incoming network or application traffic to a number of EC2 instances. It monitors the health of each of the EC2 instances associated with it and forwards traffic only to healthy instances. ELB provides a single point of contact for the EC2 instances behind ELB. Each of the EC2 instances are marked with a status, either InService if it is healthy, or OutOfService if it is unhealthy. Traffic is routed only to InService instances. ELB provides a single point of contact for the application traffic that is hosted on multiple EC2 instances. By routing traffic only to healthy instances, ELB provides fault tolerance to the application and ensures the high availability of the application. The following diagram shows how multiple EC2 instances are hosted behind ELB:
