You can think of SIM cards (and other chip cards, like bank cards) as a low power computer; they run their own embedded OS and can install applets. The applets

You can think of SIM cards (and other chip cards, like bank cards) as a low power computer; they run their own embedded OS and can install applets. The applets basically run on a tiny restricted JVM (no GC!) that follows the JavaCard profile. SIM card applications can talk with the handset through messages, asking the handset to display menus, send SMS messages, open a browser, and more [0]. The handset also forwards certain messages to the SIM card, such as certain special SMS messages. The SIM card can actually be reprogrammed over the air (slowly) using signed SMS messages. The applications that run on the SIM card can include certain banking applications (those were popular in countries with less robust banking infrastructure and wide deployments of feature phones) or multi-carrier SIMs. With regards to your card, if it gets swiped, it probably doesn't use a chip. For the chip to work, it has to be powered, which entails either being physically inserted in a slot or being powered through induction. I should probably write a blog post about all of this stuff, it's pretty interesting. Thanks for reminding me of it! [0] https://www.etsi.org/deliver/etsi_ts/101200_101299/101267/08.17.00_60/ts_101267v081700p.pdf https://www.etsi.org/deliver/etsi_ts/101200_101299/101267/08...