Load balancing is the technique of spreading work across multiple channels to achieve the best performance. One of the most common examples of load balancing is done with internet services. ISP's must use multiple servers to handle the amount of traffic from all of their users. The users only see a single point of entry but all of the inbound and outbound traffic is spread across many servers.
A Load balancer also provide a means of security. Implementing a load balancer for something like internet services means that the customer never gets direct access to a backend server. All information is passed through the load balancer.
Another key point to most load balancers is the ability to fail over. An example would be to have a load balancer spreading out internet access between two circuits whether is two DSL lines, two Cable lines or a mixture of both. If one line were to fail, all of the inbound and outbound traffic would fail over to the remaining circuit.