Digital nomads are people who are using the internet to perform their work location indepently. They're living a nomadic life of traveling, and they use their location indepent way of working to earn money to be able to travel more. A decent internet connection is basically all they need to be able to work, so they can work from nearly every place, such as their own home (some nomads stay in a particular country for several months), but also internet cafes, libraries, coworking spaces, friendly people they meet, hotel rooms,...
The type of work they mostly perform can be something like photography (selling their pictures), blogging, writing, webmaster, coding (development), ...
True that, Digital nomad myself :P