Rome has a terrific underground railway system that will get you near most tourist destinations. The Rome Metro is designed in an X shape with Line "A" and Line "B". Both lines cross at the Central Station called "Termini".

You could spend 10 days in Rome and never take a taxi - use the Metro and walk to get every where you need to go. If you will be in Rome for an extended period of time, buy a seven day Metro pass called a CIS for about 16 EURO. For shorter stays, get a day pass for 4 EURO. An individual ticket is 1 EURO.

The 'Roma Pass' is another option. For 23 Euros (as of Jan 1, 2009) it gives you 3 days of free bus and Metro travel plus free and discounted museum/monument admissions. A real bargain.  http://www.romapass.it/

You can also catch other railways from the Termini station. For example, you can transfer to the Eurostar to travel to Florence for the day from the Termini station. The Metro maps are easy to navigate. Beware at rush hours that the trains get very full of people and you'll most likely be pushed into and out of the trains. This is also the time where pick pockets make a killing - so keep your wits about you!