Size

• The primary difference between the micro SIM and mini SIM, as the names suggest, is that the micro SIM is smaller. While the mini SIM is 15 x 25 mm, the micro SIM is roughly 50% smaller, at 12 X 15 mm. Because the micro SIM is significantly smaller than other SIM cards, it can only fit into a slot designed specifically for the micro SIM.
  
  

Capacity

• While the micro SIM is smaller than the mini SIM, its capacity for data and memory is exactly the same -- up to 128K, depending on your carrier. A SIM card always holds your phone number, when applicable, and links the device that holds it to your account with a service provider. While the SIM card in some GSM devices will also record your address book information, iPhones and iPads using the micro SIM do not. These devices save contact information to an internal hard drive instead.
  
  

Parts

• The micro SIM is comprised of two parts: plastic and metal micro chip. The majority of the card is plastic, with the micro chip embedded in the card on the right side. When you insert the card into your device, it is imperative that the chip part of the SIM is laid down in the correct orientation; if it is not, the card cannot communicate with the phone or iPad.
  
  

Switching Between Devices

• Even if two devices both accept the micro SIM, you cannot swap the micro SIM card between two different types of devices. For example, the micro SIM from an iPad will not function in an iPhone 4. According to AT&T, you can swap the micro SIM card between two devices of like kind, such as two iPhone 4S phones. The next time you connect to iTunes, the new phone will need to reactivate with the new SIM.