The STLC is the sequence of phases a testing effort follows: Requirement Analysis, Test Planning, Test Case Design, Test Environment Setup, Test Execution, and Test Cycle Closure — each with entry/exit criteria and deliverables.
STLC runs in parallel with development (SDLC) and structures QA work. (1) Requirement Analysis — study requirements, identify testable items, raise ambiguities. (2) Test Planning — define scope, strategy, resources, schedule (deliverable: test plan). (3) Test Case Design — write test cases/scripts and prepare test data, build the RTM. (4) Environment Setup — prepare the test environment/data. (5) Execution — run tests, log defects, retest. (6) Closure — evaluate exit criteria, metrics, lessons learned, test summary report. Each phase has entry/exit criteria. Knowing STLC shows you understand the disciplined process behind testing, not just running tests.
For a new checkout feature: analysts clarify requirements, the lead writes a test plan, testers design cases + RTM, set up the staging environment, execute and log defects, then produce a test summary at closure.
In an interview: 'Walk me through the phases of the STLC and what each produces.'