What is Smoke testing, Sanity Testing and Regression Testing?

Smoke Testing:-

It’s done at the early phase of software development life cycle to ensure that core functionalities are working fine. The main purpose to identify the blocker or any major issues at the very early stages to save the efforts and time of the delivery life cycle.

Sanity Testing:-

It’s performed when a new changes is made on the existing code or new functionalities are introduced in the existing code base. It will test the modules or application that is newly incorporated along with the immediate impacted application. Sanity testing mainly performed after few builds when the application becomes comparatively stable but new changes has been made on that stable code base.

Regression Testing:-

It’s actually the superset of the smoke testing and the sanity testing. It is performed almost at the end of the development lifecycle to ensure the overall application is working fine or not after new release or after any changes made on the existing application. The main difference between sanity an the regression testing is that, for sanity we are covering the main changes along with the immediate surrounding impacted modules whereas for regression testing we have to cover all the modules thast is tested as part of sanity suite along with the surrounding impacted modules to ensure the application will behave appropriately.

Please refer the image link for the diagram of SDLC where these testing actually fits. This link is taken from testsigma.com website. There also this concept is mentioned in a very elaborative manner.