The Rome metro currently has two active lines, Linea A and Linea B, which intersect at Termini Station. Linea B also has a short second branch. Linea C is also in development, and has been for many, many years. It's turned into something of a joke, although the roadworks blocking some major roads aren't quite so funny.
Linea A is the primary tourist line, as it stops at the Vatican. As with all locations where crowds of tourists gather in Rome, watch out for pickpockets on the metro.
The Metro runs from 5.30am to 11.30pm Sunday to Thursday, and until 1.30am on Friday and Saturday.