There are four levels of software testing: Unit >> Integration >> System >> Acceptance.

1. Unit Testing is a level of the software testing process where individual units/components of a software/system are tested. The purpose is to validate that each unit of the software performs as designed.
2. Integration Testing is a level of the software testing process where individual units are combined and tested as a group. The purpose of this level of testing is to expose faults in the interaction between integrated units.
3. System Testing is a level of the software testing process where a complete, integrated system/software is tested. The purpose of this test is to evaluate the system’s compliance with the specified requirements.
4. Acceptance Testing is a level of the software testing process where a system is tested for acceptability. The purpose of this test is to evaluate the system’s compliance with the business requirements and assess whether it is acceptable for delivery.

Note: Some tend to include Regression Testing as a separate level of software testing but that is a misconception. Regression Testing is, in fact, just a type of testing that can be performed at any of the four main levels.