A SIM card, or subscriber identity module, is a small card in your cellphone that lets you place phone calls, send text messages, and more.

These two ways of connecting your phone to a cellular network are very different. We tell you about each and how you can switch from one to another.

If you use a Wi-Fi + Cellular version of the iPad, your tablet may come with a SIM card slot tucked into the side. In the United States, the only major carriers that use SIM cards are GSM-type ...