The main difference between a SIM card and an SD card lies in its functions. A SIM card gives your device its identity, enabling it to make phone calls and send text messages. In contrast, an SD card is essentially a portable drive that can be inserted into various electronic devices to store data.

SIM cards are crucial for mobile phone users to access cellular networks, enabling calls, texts, and data services by storing subscriber identity information. SD cards, conversely, are used to increase the storage capacity of digital devices, allowing for the storage of photos, videos, and other files.