Testing by the importance of the functions being tested 

In some sources, you can see that this classification is called «by the depth of testing». Just know that it’s the same thing. Testing by the importance of the functions being tested is subdivided into: 

1. Smoke testing. 
2. Critical path testing.
3. Extended testing. 

Smoke testing 

This is a type of testing aimed at checking the main functionality, the failure of which makes the very idea of using an application (or another object subjected to smoke testing) meaningless. 

Example 

As an example, you can use the site login form. If this functionality doesn't work, you won't have access to the site itself and won't be able to log in. 

Critical path testing 

This type of testing focuses on functionality used by typical users in their daily activities. The idea itself is borrowed from project management and transformed in the context of testing: most users mostly use some subset of application features. These features need to be tested once we have ensured that the application works in principle (the smoke test was successful). 

Example 

For example, the "Add item to the cart" function on a site. Users buy goods online by performing the same action — by adding items to the cart. It is this action that is worth checking during the critical path testing stage.

Extended testing 

This type of testing "explores" the functionality declared in the requirements. It takes into account which functionality is more important, and which is less important. Test cases can cover even the lowest-priority requirements if you have enough time.

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We continue the topic of types of testing in the next article Types of testing. Part 3.