What is MicroSIM?
A SIM card is the little chip in a GSM cell phone that holds your subscriber information.
A micro SIM, also known as a 3FF or "third form factor" SIM, is a smaller version of a SIM card that holds the same data, but is not physically compatible with larger SIM cards (2FF, "second form factor") like the ones used by mobile operators.
Micro SIM was released due to need of very small cards designed for devices whose secondary purpose would be to communicate, such as digital cameras or watches.
As Micro SIM chip is more technically advanced than one in standard SIM, contact area remains the same and backward compatibility is preserved.
The result is that the third form factor is a smaller version of the existing plug-in card, with much of the excess plastic cut away, but in such a way as not to impose restrictions on the chip size.