What exactly are the rollbacks in Higurashi? We know that the "mini-arcs" in Higurashi (When They Cry) all start with a sudden rollback. After some terrible events, the town looks normal again. 

Closer to the end of the anime we are being shown that 


   It's Hanyū who is responsible for the rollbacks, and she helps Rika keep her memories of what happened before the "rollback"


Now my question is about what those "rollbacks" really are. The wiki article, for example, says that they are 


   time travels, and what Hanyū does is reverses time to let Rika relive those days again.


However, when I was watching the anime itself, I clearly remember Rika saying that


   what they did was traveling between "worlds", trying to find a "world" where everything was right (i.e. no murders). 


Basically, if the first version is right, then 


   all the murders didn't actually happen, because the time was reversed each time. In the second case (changing "worlds"), however, all those murders would be for real in each particular world.


Was that possibly just a bad/false translation of the subtitles I was using? And which version is right, after all?
I think the correct answer lies between what you're saying and what Wiki is saying.

Here's a definition of "kakera" (usually translated as "fragments") from Umineko no Naku Koro ni (the installment of WTC after Higurashi) EP4's TIPS:


  Worlds of different fates and circumstances are called kakera, and
  witches who are able to cross the ocean of endless kakera are called
  Voyagers.


Hanyuu is taking Rika through different kakera, but she is also going back in time. If she just went to the same time in a different kakera, it might be too late for Rika to do anything. I believe towards the end of Higurashi, Hanyuu also states that her power is weakening and she can't go as far back in time.

If you're interested in knowing why/how Hanyuu has these powers, I'd definitely recommend playing the Umineko no Naku Koro ni sound novels or maybe even reading the manga. Don't watch the anime though... It doesn't cover that far into the story.