As per wikipedia:
Load testing is the process of putting demand on a system or device and measuring its response.
Load testing is performed to determine a system’s behavior under both normal and anticipated peak load conditions. It helps to identify the maximum operating capacity of an application as well as any bottlenecks and determine which element is causing degradation.
The quote in the excerpt is by Scott Barber, author of Web Load Testing For Dummies:
"Load Test - A performance test focused on determining or validating performance characteristics of the product under test when subjected to workload models and load volumes anticipated during production operations. "
There are some excellent resources for learning more about performance testing on PerfTestPlus's site: http://www.perftestplus.com/resources.htm