You'll find a schematic of major transport services at RomeGuide . There are many complexes of narrow streets and alleys. A Rome guidebook map will suffice for orientation before arrival. Make an early item of business in Rome the purchase of the most detailed map you can find. There are useful site location services, as ATAC above, on the Internet, but there are no truly useful maps that you can download and take with you.
Bus service is quite good, though often extremely crowded.
The subway is somewhat limited, but can be very useful, and fast, depending on your hotel and destination. If subway service is available it will usually be faster than bus or cab for a given trip. Easy to Walk he important parts of Rome are compactly situated. It's a great city in which to walk. Theoretically pedestrians have the right of way on a zebra.