V-Model

Introduction:
The V Model is probably the best known testing model it highlights the existence of several levels of testing and depicts the way each relates to a different development phase

Acceptance Testing:

Users perform UAT to confirm that the system does, in fact, meet their business requirements

System Testing:

It demonstrates that the system works end-to-end in a production-like environment to provide the business functions specified in the high-level design

Integration Testing:

It demonstrates that two or more units or other integrations work together properly, and tends to focus on the interfaces specified in low-level design. When all the units and their various integrations have been tested

Unit Testing:

It is code-based and performed primarily by developers to demonstrate that their smallest pieces of executable code function suitably