Testing the behavior of the whole software/system as defined in software requirements specification (SRS) is known as system testing, its main focus is to verify that the customer requirements are fulfilled.
System testing is done after integration testing is complete. System testing should test functional and non functional requirements of the software. Normally, independent Testers perform System Testing.
Validating both functional and non functional requirements of the system is called system testing.
Definition by ISTQB
Why System Testing?
#1. It is very important to complete a full test cycle and system testing is the stage where it is done.
#2. System testing is performed in environment which is similar to the production environment and hence stakeholders can get a good idea of the user’s reaction.
#3. It helps to minimize after-deployment troubleshooting and support calls.
#4. In this STLC stage Application Architecture and Business requirements both are tested.