Whether you are discussing iPads, cellular phones or other mobile devices, 3G refers to the type of cellular data network you use to connect to the Internet. The term refers to third-generation technology for telecommunications, which began with analog cellular networks in the 1970s (1G) and entered the mainstream in the 1990s with 2G and 2.5G around the world. The iPad models listed as "Wi-Fi + 3G" offer the standard Wi-Fi connectivity available on all iPads, plus the option to sign up for 3G cellular service with specific networks.
Wi-Fi vs 3G
Wi-Fi connections to the Internet require a wireless router or hotspot. You will get faster data transfer speeds on Wi-Fi, but it is not always practical to go to a wireless hotspot when you need a quick Internet connection away from your work or home wireless networks. A 3G iPad can connect to the Internet using cellular phone data signals from anywhere the data carrier -- AT&T or Verizon -- has coverage, allowing you more mobility with connectivity.
Third-generation is a blanket term for networks that feature faster speeds and advanced technology compared to 2 and 2.5G. Two major types of 3G networks are available as of April 2011: HDSPA and EVDO. Each uses a different network to carry information, and your iPad with 3G will only work on one of these networks. The iPad with 3G designed for AT&T networks uses the HDSPA technology, while the Verizon iPad uses EVDO. You must choose one carrier or the other before buying the iPad with 3G, as the configuration for the carrier network is preset and does not allow user changes.
Choosing a 3G iPad
The best 3G iPad for you depends on where you plan to use the 3G connection. Ask friends and family who use each carrier how their cellular service is in the areas where you plan to use your iPad, or check each company's coverage maps to find out where they offer service. If the areas where you travel offer better connection through Verizon, choose the EVDO/Verizon iPad. If AT&T features better service in your frequently used areas, choose the HDSPA/AT&T model. Your 3G data plan on the iPad is separate from any current cellular service you use for your phone, so you do not need to choose a carrier with which you already have an account.
Your data speeds on 3G connections vary according to many factors. You will get better data transfer when holding your iPad still where the signal is strongest. Using the 3G connection while moving, such as on a bus, significantly reduces speed. The 3G connection can also drain your battery faster than necessary, so turn off your 3G connection in the "Settings" app when you are on a Wi-Fi network or working offline for long periods of time. This way, your iPad is not wasting energy trying to process both Wi-Fi and 3G signals or searching for signals, when you don't need it.