JMeter is a tool for testing web applications and services. It simulates multiple users by generating load and measuring performance. A typical test plan in JMeter contains thread groups to simulate users, samplers to generate requests, listeners to monitor results, and logic controllers to determine flow. Elements like timers, assertions, pre/post-processors can be added to thread groups to delay requests, validate responses, and modify samplers. Once a test plan is run, results are shown in the test plan and workbench views.