ISTQB definition:
Test level (test stage) - a specific instantiation of a test process.
According to the ISTQB Foundation, there are four test levels:
Component testing (module testing, unit testing) - The testing of individual hardware or software components. Component testing: searches for defects in and verifies the functioning of software components (for example modules, programs, objects, classes, etc.) that are separately testable.
Integration testing - testing performed to expose defects in the interfaces and in the interactions between integrated components or systems. Integration testing tests interfaces between components, interactions with different parts of a system such as an operating system, file system, and hardware or interfaces between systems.
System testing - testing an integrated system to verify that it meets specified requirements. System testing is concerned with the behavior of the whole system/product as defined by the scope of a development project or product. The main focus of system testing is verification against specified requirements.
Acceptance testing - formal testing with respect to user needs, requirements, and business processes conducted to determine whether a system satisfies the acceptance criteria and to enable the user. Acceptance testing: validation testing with respect to user needs, requirements, and business processes conducted to determine whether to accept the system customers or other authorized entity to determine whether to accept the system.
Test levels can be combined or reorganized depending on the nature of the project or the system architecture. For example, there may be component integration testing after component testing and system integration testing after system testing.
While the specific test levels required for - and planned for - a particular project can vary, good practice in testing suggests that each test level has the following clearly identified:
Specific test objectives for the test level;
The test basis is the work product(s) used to derive the test conditions and test cases;
The test object (that is, what is being tested such as an item, build, feature or the system under test);
The typical defects and failures that we are looking for at this test level;
Specific approaches and responsibilities for this test level.
One additional aspect is that each test level needs a test environment. Sometimes an environment can be shared by more than one test level: in other situations, a particular environment is needed.
Human language:
Software testing is conducted throughout the entire life cycle of its development and support.
SDLC
Planning & Analysis;
Design;
Development;
Testing;
Deployment;
Maintenance.
Each stage of the software development life cycle undergoes a corresponding testing process. If you want your software to operate without any hindrance, you must conduct tests at each of these stages. Hence, there are various levels of testing.
The testing level determines what tests are carried out on — an individual module, a group of modules, or the system as a whole.
There are four such levels in total
Unit/component testing;
Integration testing;
System testing;
Acceptance testing.
Purpose
Performing testing activities at multiple levels is necessary to make sure that the product works the way it should, and it is the key to successful project implementation and delivery. It helps in the early identification of bugs and better quality of software products.
The four levels of testing shouldn’t only be seen as a hierarchy that extends from simple to complex, but also as a sequence that spans the whole development process, from the early to the later stages.
Conclusion:
So, if you are asked at an interview: What are the different test levels? The best way to answer is:
The testing level determines what tests are carried out on — an individual module, a group of modules, or the system as a whole.
In software testing, we have four different levels of testing
Unit/component testing;
Integration testing;
System testing;
Acceptance testing.