Answer.

Automated tests are divided into unit, integration, and end-to-end (E2E) to comprehensively cover the checking of the system at different levels.

History of the issue: This classification arose from the need to test applications both in parts and as a whole. Therefore, testing layers are distinguished:

specific function (unit)
interaction between parts (integration)
the entire system functioning as for the user (E2E)

Problem: Errors often arise not only in the logic of an individual method but also at the component interfaces or during the "real" launch of the entire system with external services. If everything is mixed up, it is hard to quickly localize a bug or understand where it came from.

Solution:

Unit tests verify isolated code, usually at the function level or simple classes. In advanced cases, mocks and stubs are used.
Integration tests connect several components (for example, a module and a database) to see how they work together.
End-to-end tests (E2E) simulate user scenarios — the entire path "from button to result".

Distinguishing between test types is vital to:

Minimize support costs (E2E tests are expensive)
Keep rapid feedback (unit tests are lightning-fast)
Reduce the number of false positives

Key features:

Proper distribution of tests helps build a reliable "pyramid" approach (testing pyramid).
Mixing testing styles leads to problems with bug localization.
A clear understanding of each layer's purpose leads to maximum efficiency.

Tricky questions.

Can integration tests be considered just "big" unit tests?

No, integration tests test the operation of several components together, not just individual functions. In this case, it is not always possible to use mocks — real services interact with each other.

Should all tests be end-to-end (E2E)?

No, this is an expensive and complex approach. E2E tests are only needed for critical user scenarios; most logic is covered by other tests.

Are unit tests always fast?

Not always. Sometimes it is not possible to isolate the code, or a method depends on complex external resources. In such cases, the test ceases to be a unit.

Common errors and anti-patterns

All tests are written only as end-to-end, resulting in extremely slow feedback.
Confusion in layers: a unit test starts calling the database or API, while E2E tests are built on mocks.
Only unit tests are left — problematic bugs at component interfaces are not caught.

Real-life example

Negative case

The company only covered the main functionality with E2E tests — each fix was checked overnight, tests failed unstably, and bugs were discovered late.

Pros:

Real user scenarios are covered.

Cons:

Slow feedback.
Long investigations into the causes of failures.
Many false positives.

Positive case

The team built a testing pyramid: the bottom — unit tests, the middle — integration tests, the top — only the most important E2E.