JMeter is an open source testing tool that can be used to load test functional behavior and measure performance. It was originally designed for testing web applications but has since expanded to other test functions. JMeter loads test web pages and measures performance by working at the protocol level rather than rendering pages like a browser. It can run in GUI mode or non-GUI mode from the command line. Test plans are created using an ordered tree structure and are stored in JMX files. Elements include samplers, timers, listeners, and controllers to organize test steps.