Load testing allows you to gather metrics on the performance and behavior of your application under normal or anticipated peak load conditions. This not only helps you make educated decisions about your application and environment but provides a level of confidence your users should not experience a degradation in performance or worse an outage. By being proactive and making course corrections when issues are encountered during tests.
It involves putting sufficient load on an application while it remains up and running so you are able to gather metrics about the applications performance and the environment. These metrics should include things like throughput-rates, response times, memory consumption and CPU utilization.