GPRS explain

Class B
Can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time. During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded. Most GPRS mobile devices are Class B.

The most common GPRS Multislot Classes are:

Class 2
Minimal GPRS implementation
Class 4
Modest GPRS implementation, 50% faster download than Class 2
Class 6
Modest implementation, but with better uploading than Class 4
Class 8
Better implementation, 33% faster download than Classes 4 & 6
Class 10
Better implementation, and with better uploading than Class 8, seen in better cell phones and PC Cards
Class 12
Best implementation, with maximum upload performance, typically seen only in high-end PC Cards