What Is Software Testing?

Why testing matters

Software is written by people, and people make mistakes. Testing is how a team catches those mistakes while they are still cheap and private, instead of expensive and public.

Some real failures

A few well-known examples show how far a small mistake can travel:

• Mars Climate Orbiter (1999). Ground software from one team produced thruster data in imperial units (pound-force seconds), while the navigation software of another team expected metric units (newton-seconds). The mismatch went unnoticed, the spacecraft flew far too close to Mars, and the mission was lost.
• The CrowdStrike outage (July 2024). A faulty configuration update to a widely used security product made Windows computers crash and get stuck restarting. Microsoft estimated that about 8.5 million devices were affected. Airlines, hospitals, banks and broadcasters were disrupted around the world.

You probably won’t test spacecraft. But the same idea applies at every scale.

Everyday app examples

Each of these costs something: money, support time, app store ratings, and — hardest to win back — trust.

What a manual tester does day to day

There is no single “typical day”, but most manual testers spend their time on a mix of these:

• Reading and questioning requirements. Going through user storiesUser storyA short description of a feature from the user's point of view.Open in glossary → and acceptance criteriaAcceptance criteriaConditions a user story must meet to be considered done and accepted.Open in glossary → and asking “what happens if…?” before any code is written.
• Designing tests. Turning requirements into test scenariosTest scenarioA high-level idea of what to test, such as 'User resets a forgotten password'.Open in glossary → and detailed test casesTest caseA written set of steps, data and an expected result used to check one specific behavior.Open in glossary →.
• Running tests. Installing the latest buildBuildA specific compiled version of the app or website that is ready to be installed or tested.Open in glossary → on phones, tablets and browsers and working through the checks.
• Exploring. Doing exploratory testingExploratory testingLearning, designing and running tests at the same time, guided by curiosity and a goal.Open in glossary → — using the app freely and curiously to find problems nobody thought to write down.
• Reporting bugs. Writing clear, reproducibleReproducibleA bug that happens again every time you follow the same steps.Open in glossary → bug reportsBug reportA clear written description of a bug so developers can reproduce and fix it.Open in glossary → so developers can fix issues fast.
• Re-checking fixes. RetestingRetestingRunning the same test again to confirm that a reported bug is really fixed.Open in glossary → fixed bugs and running regression testingRegression testingRe-running tests to check that new changes didn't break things that used to work.Open in glossary → to make sure nothing else broke.
• Talking to people. Joining stand-ups, pairing with developers, and helping product owners decide what is ready to ship.

Error, defect, failure: three different things

These words get used interchangeably in casual chat, but in testing they mean different things. They form a chain:

Word	What it is	Banking app example
ErrorErrorA human mistake, such as misunderstanding a requirement, that introduces a defect.Open in glossary →	A human mistake	A developer writes “greater than” instead of “greater than or equal to” in the transfer limit rule.
DefectDefectA flaw in the code or documentation that can make the software fail.Open in glossary → / BugBugA problem in the software that makes it behave differently from what is expected.Open in glossary →	The flaw that mistake leaves in the code (or document)	The app now rejects transfers of exactly $1,000, even though $1,000 is allowed.
FailureFailureWhen the software visibly does something wrong while it is running.Open in glossary →	What happens when the defect is triggered	A customer tries to send $1,000 and sees “Amount exceeds daily limit”.

QA vs QC vs testing

You will see these three terms on job ads, often used as if they mean the same thing. They overlap, but they are not identical.

• Quality assurance (QA)Quality assuranceActivities that improve the process of building software so fewer defects get made in the first place.Open in glossary → is about the process. It aims to prevent defects: good requirements, code reviews, clear definitions of “done”, agreed ways of working.
• Quality control (QC)Quality controlChecking the finished product for defects; testing is the main quality control activity.Open in glossary → is about the product. It aims to find defects in what has been built, by checking it against requirements.
• TestingSoftware testingChecking that software does what it should, and finding problems before real users do.Open in glossary → is the hands-on activity of checking the product: mostly by running it, but also by reviewing documents and designs without running anything (static testing). It is the main tool of quality control.

In practice, a “QA tester” job usually involves mostly testing, with some QA work like reviewing requirements and suggesting process improvements.

The seven testing principles

These seven ideas come from the ISTQB (International Software Testing Qualifications Board), whose Foundation Level is one of the most widely recognized testing certifications. The wording below is ours; the ideas are theirs. They sound abstract at first, but each one describes something you will run into on a real project.

Manual vs automated testing

Both are real, valuable jobs. They are good at different things.

	Manual testing	Automated testing
Who runs the check	A human	A script or tool
Great for	New features, usability, exploratory testing, visual checks, “does this feel right?”	Repetitive checks, large regression suites, running the same test on many devices, every build
Weak at	Repeating the same 500 checks every day without getting tired	Noticing that a button is ugly, confusing or in the wrong place
Setup cost	Low — you can start today	Higher — someone has to write and maintain the code

Where testing fits in development

Software is built in a sequence of stages called the software development life cycle (SDLC)Software development life cycleThe stages software goes through, from idea and requirements to release and maintenance.Open in glossary →: requirements, design, development, testing, deployment, maintenance. Testing has its own matching life cycle, the STLCSoftware testing life cycleThe phases of testing work, from analyzing requirements to closing the test cycle.Open in glossary →. Both are explained in SDLC and STLC.

In older “waterfall” projects, testing happened near the end. In modern AgileAgileA way of building software in small, frequent increments with constant feedback.Open in glossary → teams, testing happens all the time: testers review stories before a sprintSprintA short, fixed period (often two weeks) in which a team builds and tests a set of work.Open in glossary → starts, test features as soon as they are built, and keep checking until release.

Testing also happens at different levels, from small to large:

• unit testingUnit testingTesting the smallest pieces of code, like a single function, usually by developers.Open in glossary → — developers check tiny pieces of code.
• integration testingIntegration testingTesting that different parts of the system work correctly together.Open in glossary → — checking pieces work together (for example, the app and its APIAPIThe way different software parts talk to each other, like an app asking a server for data.Open in glossary →).
• system testingSystem testingTesting the complete, integrated product as a whole against its requirements.Open in glossary → — checking the whole app end to end. This is where most manual testers work.
• acceptance testingAcceptance testingTesting whether the product is ready to be accepted by the customer, users or business.Open in glossary → — checking the app meets the business and user needs, sometimes done by real users as user acceptance testingUser acceptance testingReal users or business people checking the product meets their needs before release.Open in glossary →.

We go deeper into each of these in Levels of Testing.

Mobile vs web testing at a glance

The core skills are the same, but each platform brings its own surprises.

Mobile appsWebsites

Mobile apps run on a huge variety of phones and tablets. Things to think about:

• Device fragmentationFragmentationThe huge variety of device models, screen sizes and OS versions an app must support.Open in glossary → — different screen sizes, OS versions and manufacturers (especially on Android).
• Interruptions — phone calls, notifications, switching apps, the screen locking.
• Network changes — Wi-Fi to mobile data, tunnels, airplane mode.
• Hardware — camera, GPS, fingerprint and face unlock, battery saver.
• Gestures — swipes, long presses, pinch to zoom, and finger-sized touch targetsTouch targetThe area of the screen that responds to a tap, which should be large enough for a finger.Open in glossary →.
• Testing tools — a mix of emulatorsEmulatorSoftware that imitates a real device's hardware and OS on your computer, like the Android Emulator.Open in glossary →, simulatorsSimulatorSoftware that imitates a device's behavior on your computer, like Apple's iOS Simulator.Open in glossary → and real devicesReal deviceA physical phone or tablet used for testing, as opposed to an emulator or simulator.Open in glossary →.

Websites run in many browsers on many screen sizes. Things to think about:

• cross-browser testingCross-browser testingChecking a website works and looks right in different browsers and browser versions.Open in glossary → — Chrome, Safari, Firefox and Edge don’t always behave the same.
• responsive designResponsive designA website layout that adapts to different screen sizes, from phones to large monitors.Open in glossary → — the layout should work on a phone, a tablet and a wide monitor.
• Browser behavior — back and forward buttons, refreshing mid-checkout, multiple tabs, bookmarks.
• Forms and links — validation, broken links, keyboard navigation.
• DevToolsDevToolsDeveloper tools built into browsers for inspecting pages, network traffic and errors.Open in glossary → — built-in browser tools that let you inspect elements, see network calls and simulate slow connections.