for a little explanation on the calculations (and a neat speed calculator) see:
http://www.sheldonbrown.com/gain.html
but basically, the bigger the front chainring, the harder/faster it goes. the smaller the chainring up front, the slower/easier it goes.
the reverse is true for the cassette, the smaller the rear cog, the faster/harder it goes. and the bigger the rear cog, the slower/easier it goes. some of the speeds overlap, bikes don't have as many speeds as they advertise.