The USIM is the functional equivalent of the SIM in a GSM device. It is the entity that stores subscriber-related information and implements the security functions pertaining to authentication and ciphering on the user side in UMTS and LTE (3G and 4G) devices.
One USIM may contain many profiles, each destined for certain purposes. Let's say a user has two UTMS terminals: one is of the "classic type," and the other is a multimedia terminal. When a user inserts the USIM into either of the two terminals, the subscription is the same but each terminal shows information in a different way. A different profile is used for the same subscription depending on the TE used. For example, through a multimedia terminal, the subscriber can gain access to an image archive that is not available from a classic terminal.
The SIM concept is a major innovation of GSM, which has created a paradigm of subscriber mobility that continues to be used in UMTS and LTE. The clear difference between a GSM SIM and USIM is that a USIM is, by default, downloadable and updatable through the radio path.