SIM cards are used to communicate on GSM networks. With the introduction of 3G the need for USIM cards become reality. Although it's possible to access the 3G network it with a simple SIM card, the USIM have many advantages as compared to the normal SIM card.
A USIM is a tiny computer which is able to handle several complicated applications on your handset such as e-purse, billing information etc. A handset equipped with a USIM card can also be used to make video calls, assuming the calling area is covered by a 3G network. Furthermore USIM gives better security and protects the user from unauthorized access to the phone line.
The phonebook is much bigger on the USIM, allowing thousands of contacts with information such as e-mail, secondary telephone number, address etc. Contacts in the USIM are stored much more securely on the USIM as compared to using the phone memory
