It's a bus pass valid for 10 rides on the city buses and costs 6.20€ (June 2008). Worthwhile if you are there for more than a couple of days, as individual trips cost 93 centimos. Each (person's) journey is validated on a machine upon boarding and you are permitted to share it with your friends/families.
Not so easy! At newsagents that display the sign "hay bonobus aqui". And definitely at the café in the Terminal Bus Comes near Plaza de España.