JMeter is an open source tool for load and performance testing web applications. It can test HTTP, FTP, JDBC and other protocols. JMeter simulates multiple users by using thread groups and samplers. Thread groups contain threads that represent users, and samplers define the type of requests (HTTP, FTP etc.) made by each thread. Other elements include listeners to view results, timers to simulate request delays, and configuration elements for variables and defaults. JMeter allows comprehensive performance testing of web applications and services under different loads.