The difference between __XX__ and XX method

[AON LAZIO]

Hi, Pythoners.
      I would like to know that in some class, it uses __XX__ but in some it
uses only XX
for example,

class Test:
    def __som__(self):
          ...
    def som(self):
          ...
   What does "__XX__" make the method different from XX?
   Thanks in advance
Aonlazio
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