The next tab is "Phases & Causes" is responsible for showing all the possible reasons why GC was actually started (since every garbage collections has actually a good reason to start). This is called GC Causes. We had already written a very comprehensive article about it; after that it would be obvious what are the actual reasons behind each garbage collection.
Please note that your log files might lack info about GC Causes. In that case you will see a blank graph. This is the example of how it might look like when everything is on the place:
Phases relates only to the concurrent garbage collectors (CMS, G1). They are typically a part of some big action - Old Generation GC. We provide an extremely detailed report about all of them as well as we do for each Generation. This is a typical analysis of Hotspot CMS garbage collector:
Please note that Duration here is a time when the given phase was running concurrently with the application threads. That said, Phases are not related to STW events completely.