Apache JMeter is an open source tool for load and performance testing web applications. It can simulate heavy loads on servers and networks to test their strength and analyze performance under different load types. It allows users to test performance, load test, and stress test systems to find bottlenecks and establish baselines. JMeter can test many different server types including web servers, databases, FTP servers and more. It has a graphical user interface for building test plans with thread groups, samplers, timers, listeners and other elements to test performance.