How do Smoke, Regression, and Sanity testing differ in purpose and application?

Smoke Testing — a quick set of tests that verify the main functions of the application after build or deployment. The goal is to ensure that the system starts up and the core functions work, so as not to spend time on more in-depth testing when critical errors are present.

Regression Testing — checking that new changes in the code have not broken existing functionality. Usually includes a full or partial set of tests covering old and new features.

Sanity Testing — targeted testing after minor changes or fixes to ensure that specific functionality works correctly and the bug is fixed.

In brief:

• Smoke — basic check of the entire system after build.
• Sanity — check of specific changes before more in-depth testing.
• Regression — check that changes did not break existing functionality.