MOdulator/DEModulator

Yes, that's why it's called a modem.
So, why is it of use?
Most modern cell phones incorporate a modem of some variety.
The latest phones (including the N95) have high speed capabilty and can operate at up to 3.6Mbits/sec data rates.
The network operators have enabled their equipment to take advantage of these abilities (though not all at the highest rate) and you can now find yourself in mobile broadband heaven (contention rates allowing).
Contention rates, by the way, are affected by the type of equipment in use and how efficient it is, the number of users 'online', and other prevailing conditions.
To take advantage of the phones modem you have to be connected to your PC/Laptop (a PDA can be used in the same way) by either a data cable or bluetooth.
Your settings for T-mobile are as follows -
Access point - general.t-mobile.uk
User - t-mobile or user
Password - pass
Modem initialisation string (phone and modem options in Windows) - +CGDCONT=,,"general.t-mobile.uk"
Security settings should be default or recommended and everything else OFF, unticked and unbulleted.
Set the modem transfer rate in windows as high as possible, although this will have minimal effect.
For other network operators check out a search for the settings.