What are load-balancers, and how do they work?

Load balancing is a core networking solution responsible for distributing incoming traffic among servers hosting the same application content.

By balancing application requests across multiple servers, a load balancer prevents any application server from becoming a single point of failure, thus improving overall application availability and responsiveness.

For example, when one application server becomes unavailable, the load balancer simply directs all new application requests to other available servers in the pool.
A load balancer is a device that acts as a reverse proxy and distributes network or application traffic across a number of servers. Load balancers are used to increase capacity (concurrent users) and reliability of applications.