What is a SIM card and what does it do?

The only times you ever really think about SIM cards are when you’re setting up a new phone (perhaps at a new, more affordable carrier) or traveling internationally and needing to switch to a local wireless service provider. You may know that SIMs give you access to the carrier’s network with all the stipulations stated in your plan. But if you’re looking for an actual 101 on what these little bits of metal and plastic are and how they do the things they do, you’ve come to the right place.