Barcelona has a growing metro network run by TMB, which also runs the city's bus and tram lines. There are currently eight lines: L1, L2, L3, L4, L5, L9, L10, L11. There's a free Barcelona Metro Map app to download.
Single metro tickets cost €2.15 and are valid for 120 minutes. A wide variety of passes, are available in the metro ticket machines; certain ones allow you to ride on the buses, trams, and in Zone 1 Renfe's Rodalíes de Catalunya trains.
If you lose something on the metro, ring the information number after 5pm on the day after the loss. Items found will be at the Punt TMB Diagonal information office. After a week items go to the city Lost and Found.
Several metro stations have special information points for all of Barcelona's TMB public transport:
Image by TMB