During a load test, a large number of virtual users
can be simulated from on computer. This allows the tester to effectively simulate many users without building a massive testing lab. Driver machines are often, but not always, server-scale machines equipped with a significant amount of memory and processing power.
Some common problems with Driver Machines are typically related with running out of resources. Not only CPU and Memory could be involved, it is also very well possible to exhaust network bandwidth. As a performance tester you always need to keep an eye out for potential problems with your driver machines since a malfunction here could really skew your results.
Another name for Driver Machine that is used is Agent Server.