The subscriber identity module (SIM) card installed in the 3G-capable iPad models is a micro-SIM. The micro-SIM or 3FF (third form factor) card has the same embedded hardware and contacts as other SIM cards.
The difference is that the micro-SIM is cut down to a smaller size. The SIM cards we are most familiar with in mobile phones today are actually mini-SIM cards. The iPad's micro-SIM card is approximately half the size of the iPhone's mini-SIM card.
Mini-SIM cards measure 0.98 x 0.59 inches (25 x 15 mm). The micro-SIM card inside Apple's iPad measures 0.59 x 0.47 inches (15 x 12 mm). Both cards have the same thickness of 0.03 inches (0.76 mm).