3g has to do with mobile phone and data communications.
An example of 3G is the cell network that is used for many iPhones in the US and abroad.
3g - Computer Definition
Referring to digital cellular radio systems and standards that fit under the umbrella of International Mobile Telecommunications-2000 (IMT-2000), an ITU initiative for a twentyfirst century wireless network architecture. Specifications include 128/144 kbps for high-mobility applications, 384 kbps for pedestrian speed (i.e., walking speed) applications, and 2.048 Mbps for both fixed WLL (Wireless Local Loop) and in-building applications such as WLANs (Wireless LANs). 3G systems include Universal Mobile Telecommunications System (UMTS), also known as Wideband CDMA (W-CDMA), Code Division Multiple Access 2000 (CDMA2000), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). See also CDMA2000, cellular radio, IMT-2000, ITU-T, TDSCDMA, UMTS, W-CDMA, WLAN, and WLL.
(1) (3rd Generation) The third version of a product or system. See iPhone 3G.
(2) (3rd Generation) The previous generation of cellular transmission, which features moderately high speed for Internet access compared to the very sluggish 2G systems. As 4G LTE technology proliferates, carriers maintain their 3G networks for compatibility with millions of cellphones that rely on 3G technology (for the difference between 3G and Wi-Fi, see cellular vs. Wi-Fi). Several advanced, higher-speed 3G technologies have been officially sanctioned as 4G (see cellular generations). In the U.S., the major 3G technologies are EV-DO for CDMA networks, such as used by Verizon and Sprint, and HSPA for GSM networks for carriers such as AT&T and T-Mobile. See EV-DO, HSPA and 4G.