SIM (Customer Information Module) cards really are a small nick that matches to your phone. This nick consists of the mobile phone number (if you switch it to a different phone, you’ve still got exactly the same number) as well as your username and passwords. The SIM determines the network and telephone number the mobile phone uses. These may be easily switched inside a phone, so that you can place in one for Italia, then take it off and set in a single for that US.
Another element to take into account is that many mobile phone companies “lock” their mobile phones, to allow them to simply be combined with their service. For instance, if you purchase a Orange phone, you can’t replace the SIM nick with one from O2 and also have the phone work. On unleashing your phone in How to pick Your GSM Mobile Phone below.
If you use a mobile phone having a SIM for just one country, e.g. Italia, internationally, e.g. France, you’re “roaming”. This is something you need to keep a very close eye on as it could become really expensive.