About SIM Cards
SIM (Subscriber Identity Module) cards link your phone with the account you have at your cell phone carrier. They allow one user to switch between phones (or between phones and tablets, in some cases) without having to contact their phone company and reactivate. In the United States, your ability to do this is generally limited by the carrier you're using. That is, many cell phones are locked to a specific carrier when you buy the phone from them at a subsidized discount, so the SIM card could only be used with a phone for that carrier.
They are small cards meant to be inserted into a phone. Sometimes you need to remove the battery to do this, but other times there's a small slot on the side of the phone for it. They are loosely similar in concept to a memory card (though more complex in design and with minimal storage space), as they contain a user's personal data and authentication information for connecting to the mobile network.
SIM cards contain a large scale integrated circuit. Notice the numbered pins on the diagram below, with the purpose of each one explained:
- Positive Supply Voltage
- Programming mode voltage
- I/O (Serial data)
- No connection
- No connection
GSM networks rely on SIM cards to manage subscribers attempting to use their network. CDMA phones do not use SIM cards, and all of a user's personal data for a specific phone number is stored on the phone itself. 4G LTE does use SIM cards, so you can swap the card between phones on a 4G network. That happens to be one very handy feature of 4G, particularly if you're using Verizon.