用户识别模块或SIM卡于1991年由一家名为Giesecke＆Devrient的公司出售。它们用于存储个人信息和订户详细信息，用于连接到服务。只要该设备支持特定的订户身份模块卡，就可以将SIM卡转移到其他设备。它们具有各种格式，例如32K，64K，128K和256K。
“ K”代表特定SIM卡中可用于存储的千字节内存。随着数量的增加，存储空间也增加了，并且采用了更好的Internet连接技术。最新版本(例如128K和256K)具有其他功能，可以在丢失手机时跟踪手机。较早的版本(例如32K和64K)的存储空间非常小，并且只能在卡上存储很少的消息。
32K和64K SIM卡之间的区别：
| S.No. | 32K | 64K | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1. | It was popular during the 1990s. | It was available after 2001. | |||||||||
| 2. | It is not used much in today’s world as it has become outdated. | It is used in present times. | |||||||||
| 3. | It can be used to store the contact information of lesser people of about 250. | It can be used to store the contact information of more people of about 500 (approximate value, as it depends upon the provider). | 4. | It is used for accessing 2G data services. | It is used for accessing 2G and 3G versions of data services. | 5. | It cannot be used for tracking phone in case of theft or loss. | It can be used to track the phone in case of theft or loss. | 6. | The network operators can run few mobile network operations on it because it has very less storage compared to 64K SIM cards. | The network operators can run more mobile network operations on it because it has more storage. |
| 7. | It has 32 Kilobytes of storage space. | It has 64 Kilobytes of storage space. | |||||||||
| 8. | It does not have SMS lock, auto roaming and direct access to social network accounts. | It has SMS lock, auto roaming and direct access to social network accounts. |