Club soda is water carbonated under pressure with added mineral salts, typically sodium bicarbonate and potassium sulfate, which give it a faint saline, mineral taste. That addition is what distinguishes it from plain seltzer, which is carbonated water with nothing added, and from naturally carbonated mineral water, which gets its minerals from the source rather than an additive.
In the kitchen and bar, club soda is a common cocktail mixer (gin and tonics, vodka sodas, highballs), and it's also used in cooking: its carbonation can lighten batters for fried foods, help tenderize meat marinades, and add a bit of lift to sauces and dressings.
Because its own flavor is subtle, club soda is a clean base for both alcoholic and non-alcoholic drinks, pairing well with citrus, fresh herbs like mint and basil, and warming spices such as ginger and black pepper.