Testing is the process of evaluating software to ensure it functions correctly, reliably, and as intended. It includes different testing levels such as unit testing, integration testing, and end-to-end (E2E) testing.

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Unit Testing

Tests individual functions or components in isolation.
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Key traits:
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Fast, automated, and gives immediate feedback
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Uses mocks or stubs to remove dependencies
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Tools: JUnit, pytest, NUnit

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Integration Testing

Verifies how multiple modules interact and exchange data.
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Key traits:
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Ensures components work together as intended
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Can be incremental or executed all at once
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Tools: Postman, Selenium, JUnit Integration

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End-to-End (E2E) Testing

Validates full user workflows across the entire system.
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Key traits:
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Covers UI, backend, and APIs
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Ensures real-world reliability and user experience
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Tools: Cypress, TestCafe, Selenium

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Simple Example

A login feature may go through:
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Level	What It Tests
Unit Test	Validates the login function logic
Integration Test	Verifies the login API works with the authentication system
E2E Test	Ensures a real user can log in, access the dashboard, and log out