The 3G (not G3) model allows you to connect to the Internet using a 3G cellular data connection when WiFi isn't available (you need to sign up and pay for a data plan with AT&T in order to do this, or you can use a device to connect to your existing smartphone such as a Verizon MiFi). It also includes a GPS receiver which gives you greater accuracy in determining your location when using the included Maps application (or other 3rd party apps which use location services).
Wifi and wireless are the same and are different from 3G, which is cellular. Your iMac and Macbook can connect to wifi and 3G with the appropriate hardware. The iPad wifi will never be able to connect to 3G. If course, the iPad wifi+3G can connect to both.
Wi-Fi and 3G networks are both types of wireless networks. Your iMac and MacBook have Wi-Fi capability built in. This means they can connect to the internet using Wi-Fi radio signals present at hotspots. For instance, Apple's Airport Base Station allows you to create a Wi-Fi network in your home. Naturally, you'll have an internet connection as long as you stay within range of the Wi-Fi signals (on the order of a 150' radius around the wireless router). It is within these conditions a Wi-Fi only iPad would be used.
A 3G wireless network is different than Wi-Fi. Companies such as Verizon, AT&T, Sprint, T-Mobile and others offer access to the 3G networks they have built. With a 3G network, you can access the internet anywhere the vendor's network exists. Wireless companies usually have a map on their web site indicating the extent of their network. Large cities are generally covered very well. Rural areas less so. People may choose a Wi-Fi+3G iPad to have internet access on AT&T's 3G network while traveling in a car, during business, or while camping, for example.
If you would like internet connectivity outside of the home or at hotspots, you can buy a USB modem that plugs into your MacBook. You'd then need to purchase a wireless plan from one of the network operators (typically about $60/month in the US). It is this cost that keeps me using just Wi-Fi at home and at hotspots away from home.
Changed message to indicate iPad 3G use only on AT&T's network.
I currently have a DSL hookup at home. If I were to switch to say an AT&T wireless plan with USB modem, would I be able to use that plan to connect with not only my home iMac, but also my MacBook both at home and away?
Would this wireless plan also work with the Wi-Fi 3G iPad or would I still need to pay the additional monthly $14.95/$29.95 service fee when I needed to use it?
I would not recommend ditching your DSL at home. It will almost certainly cost you more, the speeds will be MUCH slower, and you will have a limit on what you can download (and exceeding that limit can result in VERY high bills).
If you are looking at getting an iPad and want to have internet access everywhere, I would recommend getting a 3G ipad and not worrying about your macbook. If you really need wireless on your macbook, you could buy a USB cellular modem for it, but be aware that these plans are usually more expensive than the iPad plans.
Of course, you should also be aware that the 3G iPad plans are changing on Monday. The $15 plan is staying the same, but the next plan will be $25 instead of $30 but with a limit of 2Gb of data instead of unlimited. 2Gb is probably more than you would need unless you are doing a lot of streaming video while away from wifi.
Yes, a Wi-Fi+3G iPad would offer internet access at all times, assuming you're in an area covered by AT&T's 3G network. Currently, AT&T is the only provider of 3G wireless service for iPad. As Shadow99999 mentioned, AT&T's 3G plans for iPad will change on Monday June 7th.
As of Monday, an iPad 3G customer can opt for 250MB of data for $15 or 2GB of data for $25. The practical effect of this is that big downloads must be done using Wi-Fi. There are no limits on Wi-Fi downloads because they have nothing to do with use of the 3G network. Travelers using the 3G network can send a boatload of emails and do lots of web surfing without much concern for exceeding those limits. However, a user streaming video or audio (ie. Netflix movies or Pandora music) will blow past those limits in a hurry. For example, I streamed 10 minutes of a Netflix movie the other day and saw it involved 100MB of data transmitted.
Many iPad users who would like to stream video or audio are pretty steamed about the new caps on 3G downloads. I could see a person camping wanting to view a Netflix movie while in their tent. Unfortunately, they'd blow past 2GB of data downloaded using 3G in under half an hour. As mentioned above, other less data-intensive internet uses would be fine though.
