Unlike regular subscriber identity module (SIM) cards, 3G SIM cards allow your 3G-capable handset to access 3G networks, resulting in faster rates of data transmission and global roaming with the same cellular device and number.
Third Generation (3G) SIM Cards
In 2000, the International Telecommunications Union unanimously agreed upon standards and technical specifications for 3G devices, including the ability to access radio frequencies between 400MHz and 3 GHz. It set minimum data transmission speeds to between 348 Kilobit per second in a moving vehicle and 2 megabit per second while stationary or walking. The agreement also mandated compatibility with regular, second-generation systems, and flexibility to operate under different radio interfaces.
Regular (2G) SIM Cards
Second-generation (2G) devices lack universal standards or specifications. Regular SIM cards allow rates of data transmission ranging between 9.6 Kbps and 28.8 Kbps, and operate under varying radio frequencies and access protocols around the world.
In addition to providing faster data transmission, 3G SIM cards allow for global roaming using a single number and 3G-capable device. Operating under different radio protocols in different regions, regular SIM cards don't allow for such ease in global use. 3G SIM cards can access 2G networks in non-3G-capable devices, but regular SIM cards cannot access 3G networks.