What does GPRS Class A, B or C mean ?

These classes indicate how the mobile phone works:
  Class A: A class A unit can work simultaneously on the GSM and GPRS networks. Basically this means that you can make voice calls while you are connected to the Internet without any interruption in the service. There are very few mobiles on this class on the market today as these devices requires lots of CPU bandwith which would make them too expensive.
  Class B: A class B terminal can be registered on both GSM and GPRS network simultaneously but can have only one active call: you can have a voice call or a data connection at a time. Once the voice call has terminated, the data service can be resumed. Most phones on the market are currently of this class.
  Class C: A class C terminal can only be registered on one network, GSM or GPRS and cannot receive events from both network at the same time. This class is generally used by GPRS modems which are not used for voice calls.

What does Class 1, 2, ...12 mean ?

These classes describe the data bandwith supported by the device.

The basic unit of bandwith in GPRS is the channel. Each channel can transmit 8 to 12 kbps of data with the data coding scheme used today by most networks (named CS1 and CS2). New coding scheme (CS3 and CS4) will allow better data rates (14.4 to 20kbps) in specific areas. Depending on their class, a device can use to 5 channels simultaneously (the limit is 4 for a single direction: you can have 4 channels used in download and 1 upload but not 5 channels for upload and 0 for download).

Some classes (1, 2, 4, 5, 8, 9) have pre-allocated channels while others do support dynamic channel allocation (3, 6, 7, 10, 11, 12) : depending on its need, the phone can assign a direction (upload or download) to each channel.