A 3G card is a modem that allows a computing device to access the Internet wirelessly through a cellular provider's 3G network.
3G cards, which usually have built-in antennas, can be found in several form factors including internal PCI cards and external USB sticks and PCMCIA and Express cards. All such cards are collectively referred to as 3G cards when they implement 3G network standards.
In fact, 3G is not a single standard, but a combination of standardized spread spectrum radio transmission technologies and interfaces used by cellular providers to build regional wireless mobile broadband networks. These sub-standards include UMTS which is typically deployed in Europe, Japan and China, as well as CDMA2000 used in South Korea. In the United States, 3G networks currently employ either UMTS High Speed Downlink Packet Access (HSDPA) or CDMA2000 Evolution – Data Optimized (EV-DO).
To connect with the Internet, the consumer must first contract with a cellular provider and select a plan that fits their location, their data transfer requirements and their budget. A cellular provider will typically offer several types of 3G cards, allowing the consumer to select the form factor that best fits their needs. Consumers may choose an external 3G card that matches an empty USB, PCMCIA, or Express card slot in the computer they wish to connect to the Internet, or they may purchase a smartphone or tablet with a built-in 3G card.
A 3G card should not be confused with a subscriber identity module (SIM). A CMDA2000 SIM (or the UMTS equivalent USIM) is a fingernail-sized smart card which all cellular devices use to identify themselves to cellular networks. Each SIM is located inside a cellular device (be that a 3G card or an older cellphone), where it stores information such as phone number, equipment identifier and other addresses needed to connect to a cellular provider's network.