Test levels: unit, integration, system, acceptance

Unit testing checks one small piece of code, such as a single function. Developers usually write these tests. Integration testing checks pieces working together, such as a service and its database, or two services talking through an API (the way programs call each other).

System testing checks the whole product end to end, in an environment set up like the real, live one. Acceptance testing checks that the product meets the business or user need, and is sometimes done by users themselves. Each level catches different bugs at a different cost.

A unit test finds a rounding error in milliseconds, while reaching the same error through the user interface (UI) takes a minute. As a tester you will work mostly at API, system and acceptance level, but you need to know what is already covered below you.