SIM cards are tiny objects we insert into our phones and often forget about them. While they seem like they don’t do much, we wouldn’t be able to make calls, text people, or surf the internet on our phones. SIM stands for Subscriber Identity Module, which basically means it’s responsible for telling the cell towers around you about who you really are. The phone plan you pay for every month is connected to this card, so when you make a call, text, or use data, the tower will contact your provider to see if you have enough minutes, texts, or bytes remaining to complete your request.
SIM cards are usually only used in GSM phones, while CDMA phones have a built in equivalent. They often come in 3 different shapes as well: Mini/Standard, Micro and Nano. Most phones at the time of this writing use Nano SIM cards to save space in the phone for other components.
Most SIM cards can hold a couple hundred kilobytes of data, which can be text messages, contacts, or most importantly, your phone number. This makes swapping cell service between different phones simple, as all you would need to do is take out the SIM from one and place it in the other. You would wait for the new phone to register on the network and you would be good to go!
As you can see, SIM cards essential to the functioning of our smartphones, and have been a very useful invention in the past few decades.