Roman postbox. One slot is for Rome, the other for outside the city. If you're coming to Rome on holiday, you're unlikely to need to visit a post office in Rome; stamps for postcards and small letters are sold in tobacconists - wherever you see the big white 'T'.
Sending a postcard by posta prioritaria to the UK costs about €0.62c, and a few cents more to the USA.
The postboxes are red, and fixed onto the wall near most tobacconists. They have two slots: one is for Rome and the area around the city; the other is for national and international post.