A "stub" is a term used in software development to refer to an incomplete part of a software or a function. Stubs are often used as placeholders to simulate or represent a specific functionality while it's not fully implemented yet. They can be used in various stages of development, such as early planning or during the integration of different parts of software. Stubs help developers to test or develop parts of software without having all dependent components available yet.

A "mock" is a term in software development that refers to a technique where a simulated object or module is created to mimic the behavior of a real component. Mocks are commonly used in testing environments, particularly in unit tests.

Here are some key points about mocks:

1. Simulating Dependencies: In a typical software application, modules or objects may depend on each other. However, when you want to test a component in isolation without being influenced by other dependent components, you can use mock objects to simulate the behavior of these other components.

2. Simple Implementation: Mocks are often simple placeholders or stubs used to mimic specific functions or methods. They are specifically designed for testing purposes and often contain predefined behaviors to simulate certain scenarios.

3. Control Over Testing Environment: By using mocks, developers can have better control over the testing environment and simulate specific conditions or edge cases more easily. This increases the predictability and reproducibility of tests.

4. Reducing External Dependencies: Using mocks can help avoid or reduce external dependencies, such as databases or APIs, increasing test speed and making tests more independent.

Mocks are an important tool in a software developer's toolkit, especially when it comes to writing tests that are robust, maintainable, and independent of each other.