SIM is short for Subscriber Identity Module. There are two different sizes used for GSM phones, one is the same size as a credit card and the other is about the size of a stamp. Both cards contains the same eletronical circuits, it's only the plastic surrounding it that differs. The advantage of the bigger card is that is easier to swap cards, while the small card might slip between your fingers. The advantage of the small card is that it makes it possible for the manufacturer to build even smaller phones.
The SIM card is the part of the phone that contains "the real phone". It's on the SIM card that all personal facts are kept. The phone itself has no phone number, it's kept on the SIM card. In other words, you can borrow almost any other GSM phone and insert your own SIM card and make calls as usual. These calls will then appear on your own phone bill, as if made with your own phone. In the world of SIM cards you are not phone dependent, all you need is your own SIM card and almost any GSM phone. This is not all true, because some net operators or service providers sometimes give away phones for free or almost for free. To prevent the owners from swapping nets to a competetor, they use something called a SIM-lock. It means that phone is barred from the use of any other SIM card. Valid SIM cards could be SIM cards from the very same operator or service provider. But it might as well be only one single valid SIM card to your phone, and that would be the SIM card delievered with the phone. A SIM-lock works as follows, the operator has entered a lock-code into the phone, to tie it to a specific SIM card. It is fully possible to unlock the phone again. It's done by entering the lock-code again, which only the net operator has access to. So, having a SIM-locked phone means that it's tied to single card or a single operator, but not the other way around. You can use your SIM card in another unlocked phone or a phone locked to the same net operator. Using a SIM-lock makes it possible for the net operators to subcidise a phone even more, because the operator knows for sure that the clients can only use one single net service. In some countries all phones sold through a net operator are SIM-lock, for example in Hungary.
The two SIM card sizes are standardised and are the same all over the GSM world. There are many functions on the SIM cards like, for example, memory space to save maybe 100 names and phone numbers and maybe 15 SMSs, short text messages. We can anticipate SIM cards with memory enough for maybe 200 names an phone numbers an still more SMSs. You can activate many different services on the SIM cards. Some net operators take advantage of this, for example in the UK. You can market budget subscription with less functions. There are advantages and disadvantages to this. One advantage could be beeing able to have a very simple service just in case of emergency, but it's a step backwards for the developement of mobile phoneing. After all, isn't it a bit of the charm to have all these gadgets and services?
One function that might come in handy is to be able to bar the SIM cards from making calls to other phone numbers that those kept on the card, but still beeing fully able to receive calls. This could be of great use to parents who wish to be able to call their youngster and at the same time make it possible for them to call home but without risking any enormous bills. The youngsters can only call the numbers not barred by the parents and thus making it impossible for the youngsters to make long and expensive calls to their friends.
The developement of SIM cards is as fast as everything else in the mobile phone business. The SIM cards are becomming more and more sofiticated and more and more functions are beeing added and improved.
Nowadays there are so called PrePaid SIM cards. This means, you buy and use a SIM card almost the same way as you buy and use a phone card for a public phone. The SIM cards are charged with a certain amount of money and has a phone number of its own. Thus, you can receive calls, too. When you have consumed your credit, you can add more to the card. If you don't use the card or don't recharge it within a certain given period, the SIM card will stop functioning. This type of "subscription" has become vey popular and is responsible for a big part of the new clients.
On the GSM market of today you can find so called SIM-readers. They will connect to a COM-port on a PC-computer. This makes it possible to read and alter the contents of the SIM cards, from the keyboard of the computer. It makes it more convenient to handle the names and phone numbers as well as the text messages in the memory of the SIM cards. You simple read the phone list and has it on the computer screen. There it is very easy to add and alter whatever you wish. It is considerably easier from the keyboard of the computer than from the phone. On top of that, you can store the phone book and the text messages on your computer, for future use. Have a look at a product from the German company Towitoko, "SIM-Leser".