Seltzer vs. Club Soda vs. Perrier:
All are clear beverages. Seltzer has bubbles added, but no salt. It's made by adding carbon dioxide gas to plain water.
Club soda (aka soda water) has bubbles (usually a bit less than seltzer) AND sodium (or an acid, or both) added. It's made by adding sodium bicarbonate to water, which results in bubbles of carbon dioxide -- and sodium left dissolved in the water.
Perrier doesn't need to have bubbles added; the water is naturally effervescent.
Any of them may also have flavorings added. And I detest the lot of 'em.

I prefer my water bubble-free.