Load testing is the process of putting increasing demand on a system or software until the system or software breaks or fails to function. The purpose of load testing is to test the system against unusually high usage patterns to determine how the system or software will react.
One example of the need for this type of testing is a web server hosting a web site that is seasonal or receives peak levels of web traffic during highly publicized special events. Another example would be a link to a small web site from a very popular site, that then doubles or triples the number of concurrent visitors to the smaller web site. Load testing in each example could determine ahead of time whether each site could handle the increased traffic without the hosting server failing to serve web pages to visitors during the peak load.