An RO unit is a specific type of filter that removes nearly all of the impurities from the water, giving you something resembling distilled water. Water filters otherwise vary considerably, depending on what you go for.
RO's work by prefiltering the water and then passing it, under pressure, across a membrane. As it does this only the pure water stays put and about 7 times as much water goes down the drain more concentrated.
If you do get one you'll need to add minerals back in, some people use an RO/tap water mix, I personally use a remineralisation powder.
Main question with regards to better water for you fish is, what are you keeping, followed by what is your current water like. The more picky soft water fish will benefit from it, especially if you have hard tap water, but most common simple fish won't care and some, such as the malawi's will not be happy at all. It's regularly used for marines as they want the most reliable mix from the salt they are using, and RO is reliable.
Buying them depends where you are. I'm guessing UK given you username. I got mine from osmotics UK, but there are several other good suppliers.
