Refresco vs. gaseosa
The Spanish language, rich in vocabulary, has different words that refer to 'soft drink' or 'soda'. These words can change depending upon the Spanish-speaking country. The words 'refresco' and 'gaseosa' both generally refer to a type of carbonated soft drink, but they are used differently in various countries.
Refresco
This term is broadly used in Latin America and Spain and it refers to any kind of soft drink, which can be carbonated or not.
Vamos a tomar un refresco en la plaza central.
(Let's have a soft drink at the central square.)
Prefiero el refresco de naranja en lugar del de manzana.
(I prefer the orange soft drink instead of the apple one.)
El refresco de cola es muy popular en todo el mundo.
(Cola soft drink is very popular worldwide.)
Gaseosa
This word is more specific, typically referring to carbonated drinks, like sodas. Gaseosa is most commonly used in Spain.
Voy a comprar una gaseosa para la cena.
(I'm going to buy a soda for dinner.)
¿Te gusta la gaseosa de limón?
(Do you like lemon soda?)
No me gusta la gaseosa, tiene demasiado gas.
(I don't like soda, it has too much fizz.)
Summary
While both 'refresco' and 'gaseosa' can be translated as 'soft drink' or 'soda', there are regional differences in how they are used and what exact beverage they reference. 'Refresco' is a more general term used throughout Spanish-speaking regions for any type of soft drink. On the other hand, 'gaseosa' tends to specifically refer to carbonated drinks and is most commonly used in Spain.