Keith Combs has a great post providing a high-level technical overview of what DirectAccess in Windows 7 Enterprise is and how it works.
The great thing about this feature is that it removes the VPN requirement to access corporate resources when on the road. Because you computer never leaves the network, IT admins are free to administer the mobile asset as if it were on the network. From an end-user perspective you also gain the peace of mind know that you can still have your personal browsing habits at home on a company machine without putting your company at risk. Of course, I wouldn’t recommend looking at porn on a company machine, but at least you know that if you do, it will be using your own internet connection as opposed to your companies. :) Realistically, what this means is that companies no longer have to invest a ton of money in a VPN solution along with the associated bandwidth overhead. Since most consumers have better internet connections at home than they do at work, end-users won’t have to burden their work infrastructure when they are trying to work as well as watch a show on BBC iPlayer!
Read the technical documentation