Thread group: For any test plan, thread group is the beginning part of JMeter elements. It is an important element of JMeter, where you can set number of users and time to load all the users given in the thread group. Thread Groups is a collection of Threads. Each thread represents one user using the application under test. Basically each Thread simulates one real user request to the server. For example, if you set the number of threads as 100; JMeter will create and simulate 100 user requests to the server under test.