There is a wide range of bicycles available for any type of riding you plan to do. If you are just getting into cycling, choosing a bicycle that allows you to ride in a diverse range of environments, rather than purchasing a specialty bike, will likely motivate you to keep riding and to develop as a cyclist.

And unless you plan to own a different bicycle for each type of riding (which is certainly not a bad thing!), choosing a single versatile bike will allow you to spend more time pedaling and less time thinking about pedaling.

As your commitment to cycling deepens, you may have the desire to explore more specialized cycling. The following resources will help you understand the many cycling choices and determine which bicycle will fit your lifestyle and interests.  

A comprehensive guide to bicycle types:
http://www.bikesrnottoys.com/diffBikes.htm

A summary of bicycle types:
http://www.primusweb.com/fitnesspartner/library/equipment/buybike.htm