I read somewhere that using bottled or purified water can be REALLY bad for fish...is this true?

There is a girl on another board that I'm on who has had a betta for a little bit now and she uses bottled spring water for her fish (he's in a tiny tank and I can't convince her otherwise, she changes his water often though).

Is this something bad or is it okay?

I wouldn't use bottled water. It contains minerals and possibly other stuff that can harm your fish. I don't know what type of tap water you have, but as long as you treat it with additives or conditioners there shouldn't be any problems with it.

From what i understand, although I am no expert, is that you do not want to use distilled water or some types of bottled water because they are missing essential minerals and electrolytes that the fish need.

Some kinds of bottled water are simply purified tap water that has been filtered, and I have been told that is ok to use (but can be very expensive). But other types of bottled water have all the "good" stuff removed and sometimes have yucky stuff added. I think unless you know you have serious problems with your tap water, it is best to use that with a good water conditioner like Prime or Amquel.

My sister has had a betta for over a year now and swears by distilled water. Don't know if that is right but it works for her

I agree, for the vast majority of people dechlorinated tap water of a similar PH and GH and temperature is absolutely fine for water changes.