Is there any difference between たすける and すくう or are they synonyms for each other? I heard somewhere that 救う is used when a second person is talking about being rescued but I'm not sure.
I can confirm from anime listening experience that 救う is rather used for saying that one's been saved by someone... e.g あいつに救われた、とかな...
But I don't think they're necessarily synonyms, they seems to hold different levels of implication for the action of saving/rescueing. 助ける is more of a general use for expressing the action of helping/saving while 救う implies the fact of rescueing someone/being rescued FROM/OUT OF a precise situation...
Medyrius covered it. It's the same difference as "to save" and "to rescue", interestingly. The first is much broader while the second is very specific.
From what I understand 救う has a more serious feeling to it, and can also be used in religious context (救われた, same as English when someone says they are 'saved'). I think 助ける is just a nice and safe general sort of term that can even be used in casual situations.