Built in collaboration with Clarius and Instedd, the library was created mainly for developers who aren't currently using any mocking library (or are displeased with the complexities of some other implementation), and who are typically manually writing their own mocks (with more or less "fanciness"). Most developers in this situation also happen to be quite pragmatic and adhere to state (or classic) TDD. It's the result of feeling that the barrier of entry from other mocking libraries is a bit high, and a simpler, more lightweight and elegant approach is possible. Moq achieves all this by taking full advantage of the elegant and compact C# 3.0 language features collectively known as LINQ (they are not just for queries, as the acronym implies).

Moq is designed to be a very practical, unobtrusive and straight-forward way to quickly setup dependencies for your tests. Its API design helps even novice users to fall in the "pit of success" and avoid most common misuses/abuses of mocking.

Currently, it's the only mocking library that goes against the generalized and somewhat unintuitive (especially for novices) Record/Reply approach from all other frameworks.

Not using Record/Reply also means that it's straightforward to move common expectations to a fixture setup method and even override those expectations when needed in a specific unit test.

You can read more at Moq Home Page