various testing levels code example

Example: testing hierarchy

1- Unit testing:
Developers test each module or block of code during development.

2- Integration Testing is testing 
Combine all of the Functionalities. To answer
When I integrate them, can I still use all of the functions?
Make sure they all still work.
3- System Testing is End-to-End testing. 
Test everything from beginning to end.
4- Acceptance Testing is Hire a UAT (User Acceptance Testing) Team or
Business Analyst can also do Acceptance Testing.
After testing has been complete you have to get another team to do 
acceptance testing so they can confirm the QA teams
testing was successful and have the product ready for the customer