End to end testing is the methodology to validate whether the flow of application from the starting point to end point is happenning perfectly.
End-to-end testing is a technique used to test whether the flow of an application right from start to finish is behaving as expected. The purpose of performing end-to-end testing is to identify system dependencies and to ensure that the data integrity is maintained between various system components and systems.
For example, while testing a web page, the start point will be logging in to the page and the end point will be logging out of the application. In this case, the end to end scenario will be
log in to the application, get into inbox, open and close the mail, compose a mail, either reply or forward the mail, check in the sent items, and log out.