Software testing is an important part of the software development process. In normal software development there are four important steps, also referred to, in short, as the PDCA (Plan, Do, Check, Act) cycle.
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Let's review the four steps in detail.
Plan: Define the goal and the plan for achieving that goal.
Do/Execute: Depending on the plan strategy decided during the plan stage we do execution accordingly in this phase.
Check: Check/Test to ensure that we are moving according to plan and are getting the desired results.
Act: During the check cycle, if any issues are there, then we take appropriate action accordingly and revise our plan again.
So developers and other stakeholders of the project do the "planning and building," while testers do the check part of the cycle. Therefore, software testing is done in check part of the PDCA cyle.