Vaso vs. copa vs. taza
In Spanish, the words vaso, copa, and taza all refer to types of drinking vessels, but they are not interchangeable. The choice depends mainly on the shape of the vessel, its material, and the type of drink it is used for.
Vaso
A1A vaso is a standard glass or tumbler. It typically has straight sides, no stem, and no handle, and is primarily used for cold drinks like water, juice, milk, or soda.
Me gustaría un vaso de agua fría, por favor.
(I would like a glass of cold water, please.)
El niño derramó su vaso de leche sobre la mesa.
(The child spilled his glass of milk on the table.)
Para la fiesta, compramos vasos de plástico.
(For the party, we bought plastic cups.)
Sirvió el refresco en un vaso alto con mucho hielo.
(She served the soda in a tall glass with a lot of ice.)
Copa
A2A copa is a stemmed glass, such as a wine glass, champagne flute, or a glass for cocktails like a margarita. This word also means "trophy" or refers to a serving of dessert, like ice cream, in a specific bowl.
Brindemos con una copa de vino tinto.
(Let's toast with a glass of red wine.)
El camarero pulía una copa de champán.
(The waiter was polishing a champagne flute.)
Mi equipo de fútbol ganó la copa el año pasado.
(My soccer team won the cup last year.)
¿Te apetece una copa de helado para el postre?
(Would you like a dish of ice cream for dessert?)
El barman preparó el cóctel en una copa de martini.
(The bartender prepared the cocktail in a martini glass.)
Taza
A1A taza is a cup or mug, almost always made of ceramic or porcelain and featuring a handle. It is used for hot beverages like coffee, tea, or hot chocolate. It is also used as a unit of measurement in recipes.
Cada mañana, bebo una taza de café bien caliente.
(Every morning, I drink a very hot cup of coffee.)
La receta necesita una taza de harina y media de azúcar.
(The recipe needs one cup of flour and half a cup of sugar.)
Me regaló una taza con un dibujo de un paisaje.
(He gave me a mug with a drawing of a landscape.)
En una tarde de lluvia, disfruto una taza de té.
(On a rainy afternoon, I enjoy a cup of tea.)
Le preparó al niño una taza de chocolate caliente.
(She prepared a mug of hot chocolate for the child.)
Summary
In short: use vaso for a simple, stemless glass for cold drinks like water or juice. Use copa for a stemmed glass for alcoholic beverages like wine or cocktails, or for a trophy. Use taza for a mug with a handle for hot drinks like coffee or tea, or as a measuring unit in cooking.