Is a pickled cucumber just a pickle?
Yes.  Basically, it is pickled in a brine, vinegar, or other solution and left to ferment.
How long does that process take?
I'm really not sure about that but they are only known as pickles in the US and Canada.  Other countries call them gherkins.  
What else do you know about them?
Sometimes cucumbers are brined which is a fermentation process that makes them grow sour.
Can they get too sour?
Probably if it is not prepared correctly.  Brining requires a lot of salt too.  I'm not a fan of that.