A rudimentary knowledge of Catalan is certainly helpful in getting around Barcelona, as most addresses and road signs are written in Catalan not Spanish. Particularly helpful to know is that carrer = road (calle in Spanish), plaça = square (plaza in Spanish), passeig = passage (paseo in Spanish) and avinguda = avenue (avenida in Spanish). Entrada and sortida are entrance and exit, no fumeu means no smoking and obert and tancat mean open and closed respectively.
But first a little introduction. Catalan is an entirely separate language to Castilian Spanish, and not just a separate dialect as many assume. It is spoken by around 9.5 million people and as well as 6 million speakers from Catalonia, 2.5 million Valencians speak a regional version of the language, whilst Catalan is also the preferred tongue of residents of the Balearic Islands and the official language of Andorra. Those who were forced to learn latin at school will be pleased to have a small grounding in Catalan already, as it is a so-called Romance language derived from Cicero and chums. For modern speakers, think of a mix of Spanish and French with a bit of Italian thrown in and you're half way there.
Here are some phrases and beginners' vocabulary to get your tongue around, below. Meanwhile, if you're keen on learning some Catalan then head to our page on Catalan and Spanish language schools in Barcelona for some suggestions on where to study.