Some people may get confused when first introduced to Selenium RC or Selenium Grid. The best answer to this is what i read on Selenium users group by Haw-Bin:

It's possible, but it's not a great usage of Selenium. One reason is that actually starting that many browsers will create a lot of overhead on the machines they are running on. You'll end up wasting a lot more computing power than you have to.

It's common among test engineers to mix between performance testing and stress testing. It's worth to mention that the difference between the two lies in how they are executed:

Performance testing (aka load testing) is testing that is performed, from one perspective, to determine how fast some aspect of a system performs under a particular workload. [source]. A very simple analogy is by imagining yourself holding a stopwatch while your work colleague is performing certain navigations and mouse clicks on a web application. Your job here is to take time measures for the transactions. Then compare them to the required figures.

The purpose of performance testing is to make sure that the system is running according to the required response time performance specifications under a particular load. And to reveal the system time bottlenecks and thresholds.

The challenge in performance testing is to achieve a workload for the testing environment that is a close match to that of production environment.