JMeter is an Open Source testing software. It is 100% pure Java application for load and performance testing on different protocols or technologies. JMeter simulates a group of users sending requests to a target server, and return statistics that show the performance/functionality of the target server / application via tables, graphs etc.
It was developed by Stefano Mazzocchi of the Apache Software Foundation. He wrote it primarily to test the performance of Apache JServ (Now called as Apache Tomcat project). Apache later redesigned JMeter to enhance the GUI and to add functional-testing capabilities.