I Recently got my first ever SIM card of my life. I hadn't got the body for my SIM so was keeping myself relieved by just seeing the SIM. On observing the SIM, i just came out with a question "What does SIM really means?". As soon as the idea came, i searched on internet for the answer and the answer was simple Subscriber's Identity Module(SIM) and the name really has its meaning too. I am not describing the whole history of that but giving here a brief description of it.

A Subscriber Identity Module (SIM) on a removable SIM Card securely stores the service subscriber key used to identify a subscriber on mobile telephony devices (such as computers and mobile phones). The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone or broadband telephony device.

SIM cards are available in two standard sizes. The first is the size of a credit card(85.60 mm × 53.98 mm x 0.76 mm). The newer, more popular miniature-version has a width of 25 mm, a height of 15 mm, and a thickness of 0.76 mm. However most SIM cards are supplied as a full-sized card with the smaller card held in place by a few plastic links and can be easily broken off to be used in a phone that uses the smaller SIM.

Each SIM Card stores a unique International Mobile Subscriber Identity (IMSI). The format of this number is as follows:

• The first 3 digits represent the Mobile Country Code (MCC)
  
• The next 2 digits represent the Mobile Network Code (MNC)
  
• The next 10 digits represent the mobile station identification number.

Now a days SIMs are available for GSM and CDMA handsets. I think GSM is the best as u can remove the SIM from your mobile handsets and get it into your friend's one.

Hope this post will add up to your knowledge as of mine. You can share some more information with me if you have by posting some comments.