Software testing is the process of evaluating the product or software of the client or business analyst based on their requirements.
Software testing is the testing and releasing a product based on requirements without bug and defect.
Software development life cycle includes requirement gathering and analysis, system design, implementation and or developing coding, testing, deployment (releasing the product).
It is a process of correcting and checking that the software is meeting expectations of the clients or stakeholders without ant bugs.
Further the software from error (bugs) , need to add requirements are to be maintained by tester. The activities of the tester includes need to monitor the product free from error and if need to add new requirements.
Software development life cycle model involves :

1. waterfall model
2. v- model
3. Agile model
4. spiral model
5. WATERFALL MODEL- it is an sequential model, in this each phase must be completed before nextphase.
6. V-MODEL- in this model during testing phase if any bug occurs we can go back and able to correct the bugs.
7. AGILE MODEL-here the testing is continous process
8. SPIRAL MODEL- it focus on risk analysis and it combines elements of both iterative and waterfall model.