what is club soda?

I'm from spain and see a lot of times Club Sod ain cocktails recipes but I don't know what it is. Does it means 7-Up?

No, I think it's just carbonated water.

I believe Club Soda is carbonated water with a sodium content. While seltzer water is just plain carbonated water.

From the wikipedia.org listing for Club Soda:

Club soda is a soft drink closely related to soda water, except that it contains a small amount of salt for flavor. Sometimes, however, club soda is used as a synonym for soda water; see that article for details.

The composition of club soda varies among different brands. The most common additions are sodium citrate and sodium chloride; other possible ingredients are sodium bicarbonate, potassium sulfate, and disodium phosphate. The total amount of sodium is usually comparable to other flavored soft drinks.

Club soda containing sodium citrate has a slightly sour flavor (similar to citrus fruits), noticeable when consumed by itself and in certain mixed drinks, but usually not strong enough to be detected when mixed with stronger flavored beverages, such as juice. It is often used as a substitute for plain soda water, commonly as a mixer.

Common mixed drinks which may include club soda:

Scotch and soda
Whiskey sour
Popular brands of club soda:

Canada Dry
Schweppes

Agua con gas.