Your smart phone may contain a SIM Card. What is a SIM Card, what does it do ?
A SIM (Subscriber Identity Module) Card is essentially a memory chip.
Each SIM Card contains a unique serial number plus an internationally unique number of the mobile user. Each one is personal to the user and the service provider a (ie a Bell SIM Card will only work on the Bell Network). The smart phone must also be compatible ie HSPA (or UMTS) and 850 MHz and 1900 MHz compatible).
It is possible to use your card in someone else's cell phone, the charges, if you are using a Bell SIM Card, will be charged as if you had made the call from your own phone via the Bell Network, which is good news for the person letting you use their SIM enabled smartphone.
This is possible because the SIM Card keeps stores your phone number, your contact phone numbers and text messages as well as your account information -- it becomes your mobile identity, the information is stored on thee memory chip that acts like a mini hard drive.
The SIM Card can be moved from one device to another, providing all your devices are compatible, there should be need for multiple SIM Cards.