A nomad is someone with no permanent home, like the member of a tribe that moves from place to place in search of food, or the rock star who spends 365 days a year in tour buses and hotel rooms.
The word nomad has traditionally meant a member of a tribe of people who roamed from place to place in search of animals and shelter. Nomadic tribes wandered out of necessity. Wherever the food went, they went. In its more modern sense, a nomad is someone who would rather wander from place to place than set down roots, like the nomad who crashes at his friends' apartments in various cities instead of renting his own place.