Cerveza vs. birra
In Spanish, both cerveza and birra mean "beer", but they are used in different contexts. The primary difference between them lies in their level of formality and regional prevalence.
Cerveza
A1This is the standard, universal, and formal word for "beer" in Spanish. It can be used in any situation, from ordering at a fancy restaurant to talking about beer in general, and it is understood in every Spanish-speaking country.
Me gustaría pedir una cerveza de barril, por favor.
(I would like to order a draft beer, please.)
La producción de cerveza en esta región es famosa mundialmente.
(The production of beer in this region is world-famous.)
¿Cuál es tu marca de cerveza favorita?
(What is your favorite brand of beer?)
Este plato combina muy bien con una cerveza ligera.
(This dish pairs very well with a light beer.)
Vamos a comprar un paquete de seis cervezas para la reunión.
(Let's buy a six-pack of beers for the get-together.)
Birra
B1This is a very common, colloquial, and informal term for "beer", similar to saying "brew" or "a cold one" in English. It is borrowed from Italian and is used heavily in countries like Argentina, Uruguay, and Spain, almost exclusively in casual settings among friends.
Che, ¿vamos a tomar unas birras después del partido?
(Hey, shall we go grab some beers after the game?)
¡Qué calor! Me apetece una birra bien fría.
(It's so hot! I'm craving a really cold beer.)
Traje un par de birras para compartir.
(I brought a couple of beers to share.)
En ese bar tienen la mejor birra artesanal de la ciudad.
(That bar has the best craft beer in the city.)
Anoche nos quedamos en casa viendo películas y tomando birra.
(Last night we stayed home watching movies and drinking beer.)
Summary
In essence, always use cerveza as your default word for "beer"; it is always correct and universally understood. Use birra only in very informal, relaxed situations with friends, particularly if you are in Spain, Argentina, or Uruguay, to sound more like a local. Think of cerveza as "beer" and birra as "brew".