Sparkling juices are juices that have been carbonated make them fizzy.
The more expensive sparkling juices use natural carbonation. The juice is let ferment a bit to create natural carbonation, then it is flash-pasteurized. This makes it somewhere in between wine and juice.
The cheaper ones are artificially carbonated with C02, with the juice flash-pasteurized immediately after pressing so that no fermentation will take place. European-made ones may advertise themselves as “low in alcohol” (around 3% alcohol) as opposed to the “100% alcohol” free more common in North America.
The more carbonated any beverage is, including juice, the less sweeter it seems.