What is end-to-end testing?

Exploring the application's behavior by running the entire application from end to end and also it verify the software system along with its sub-system.
And for doing the same one should have enough knowledge of the whole system to be tested is to exercise a complete production like scenario.
Like from start to finish for example Login and logout and its helps in testing the integration with external applications.
It checks a complete flow of the application and is also called Chain testing.
 
  The purpose of carrying out end-to-end tests is to ensure that the right information is passed between various system components and to identify system dependencies.
End-to-end testing is a methodology used to test whether the flow of an application is performing as designed from the starting to finish.
end-to-end testing is to test whether the flow of an application, from start to finish, is as expected.
To ensure that the right information is passed among all application components.
End to End testing is usually executed after functional, Unit testing and system testing.It tests the complete scenarios the entire app while using the application that the user will execute.

Preparing Test cases.
End-to-end requirement tracking.
Checking with all interfaces and backend database.
For acceptance testing Selenium was used earlier & it has a lot of bugs and is quite error prone.
For Selenium/UI Browser testing an end to end testing has been frequently confused and incorrectly used as synonym.
create regression test plan for all identified application input and output.
Is often considered one of the least popular testing is End to end or Acceptance testing.

End-to-end testing used to test an application from the beginning to end in software testing is a technique whether it is behaving as expected...
Hope it helps you a lot in understanding.