While network application functionality depends on the specific mobile network, their core features are quite similar: store network parameters securely and identify to the network, as well as authenticate the user (optionally) and store user data. A UICC can host and run more than one network application (hence 'universal'), and thus can be used to connect to different networks. The physical smart card is referred to as Universal Integrated Circuit Card (UICC) and different mobile network applications than run on it have been defined: GSM, CSIM, USIM, ISIM, etc. While the original GSM standard did not make a distinction between the physical smart card and the software required to connect to the mobile network, with the introduction of 3G standards, a clear distinction has been made.
Of course, not every smart that fits in the SIM slot can be used in a mobile device, so the next question is: what makes a smart card a SIM card? Technically, it's conformance to mobile communication standards such 3GPP TS 11.11 and certification by the SIMalliance. In practice it is the ability to run an application that allows it to communicate with the phone (referred to as 'Mobile Equipment', ME, or 'Mobile Station', MS in related standards) and connect to a mobile network.
