You induce a normal or expected workload to a system under test and observe it. Load tests are used to verify your quality criteria.
You want to learn when your system breaks and how it starts to fail when being in a serious traffic situation.
How effective can I grow? You can run a series of stress tests and gather data on how effective you really are.
Spike Testing can tell you if you are making good use of the elasticity of the cloud when faced with these kinds of events.
You hold the load over longer periods of time to look for long term effects, like memory leaks, disk space filling up, etc.
Configuration testing is, a very important tool in order to learn about the impact of a system's environment to its performance.
The idea of Availability & Resilience Testing is to look at certain processes and behavior under load.