Sants Estació: pretty much every commuter train, including those for Sitges and the airport (for leaves from here, along with the bulk of national and international trains, and many national bus routes.
Passeig de Gracia: the most central station for getting to the airport by train. (Though we prefer the Aerobus from Plaça Catalunya).
Estació de França: From this gorgeous 19thC station, local trains depart to the coastline south of Barcelona, including popular destinations such as Tarragona and Port Aventura. You buy tickets here for any train on the RENFE network, and queues tend to be shorter than either Sants or Pg. de Gràcia.
Plaza Catalunya and Plaza Espanya: these two stations serve all of the Catalan Government Railways (FGC) routes. Go to Espanya if you want to go to Montserrat.
Estació del Nord: serves the Ryanair bus to Girona, as well as the majority of national and international bus routes. It’s located beside Arc de Triomf metro station.
In Sants and Estació del Nord you’ll find left luggage lockers – handy if you have loads of time to kill before your flight.