JMeter is an open-source load testing tool that can test various server types including web servers. It allows performance testing by simulating a heavy load on a system and stress testing to push a system to its limits. Key benefits of JMeter include its ability to test HTTP, database, JMS, mail protocols and more. It also has a full multithreading framework and customizable plugins. Creating a test plan in JMeter involves adding thread groups to simulate users, HTTP request samplers, listeners to view results, and other elements like timers, assertions and post-processors. JMeter also supports recording tests from a browser and distributed testing across multiple machines.