I work for a company that designs, builds, and deploys 4G wireless. I actively test these networks. For years when people ask me what I do, I say I work on WiMax and LTE which you have never heard of, but one day you will. That day has finally come!
4G is a loose technical and marketing term to describe a vastly superior wireless network. The ITU is an international standards body that nobody cares about. They made waves by inventing their own measuring stick to call 4G, and have since backed off those claims and now include Verizon, AT&T, Sprint/Clear, and even T-Mobiles new networks as 4G.
From a consumer real world usage point of view, here is what you will experience. Voice will sound the same, if not ever so slightly worse sounding. Your internet browsing, downloading, and data usage will be faster, maybe 2 times as fast as what you experience now. Your monthly fees will either stay the same, but with less data included, or increase in price. Your battery life, at least until the nation is blanketed with 4G coverage, will suffer.
Why are companies investing billions in upgrading to 4G? In the under developed world (and 50% of the world has never made a phone call) the 4G networks don't need the traditional cell towers and equipment rooms that have to be air conditioned to work. Purchasing land to build and maintain towers is a limiting factor, and throwing one of these on top of a 2 story building or even the ground in the desert and having phone and internet access is huge. 4G will help connect the rest of the world.
From a US/Europe/parts of Asia point of view, 4G is all about cost savings. There aren't as many patents that you have to pay royalties on to companies. Current phones have a huge percentage of their cost going toward these patent holders. The networks can be installed at existing sites, making them cheap and easy to setup. They are extremely efficient, from a voice and data perspective they can get more people on their network using less resources. They also will be able to provide services they couldn't before, such as home phone replacement, which comes with more fees and profit.
How fast is fast? I'm a poor judge because I use the network in a perfect environment. I can easily reach speeds over 30 megabits. Go to speedtest.net and test your connection now. It takes 2 minutes to see how fast your home internet is. The real world speeds for 4G will probably be 5 megabit to download and 1-2 megabit for upload. Not as fast as the top of the line home internet connections, but for going 60 miles an hour in a car, that certainly is much faster than anything available today in a phone.