[Flutter] InkWell vs GestureDetector: what is the difference?

I'm completely new to Flutter and found out about InkWell and GestureDetector. It seemed to me that they are almost the same. The official documentation doesn't provide any in-depth comparison between them.

1. 
   What are the differences between InkWell and GestureDetector?
   
   
2. 
   When to use which?
   

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