Quality Assurance: It is nothing but building adequate confidence in the customer that the developed software is acceding to requirements. THE entire Software Development Life Cycle (SDLC) comes under QA. It is process-oriented.
System Testing: It is the process of executing an entire system i.e. checking the software as well as parts of the system.
Examples of quality assurance activities include process checklists, process standards, process documentation, and project audit. An example of System testing might check that every kind of user input produces the intended output across the application.