A Drift is a community of ships that move aimlessly through space (hence the name). Their inhabitants are typically referred to as "Driftors".
They are most easily compared to slums, and are commonly stricken with poverty and small-scale crimes.
Drifts are typically created after a species is forced to leave its planet, such as due to overpopulation, or following a global-scale conflict (as was the case with humans). In the latter's case, most of the ships that make up drifts are military cruisers that have been imperfectly re-fitted for civilian housing. Because such ships were never meant for civilian use, they are more often than not very cramped; it's not uncommon to share living quarters with multiple other families.
There is a harsh stigma against drifts (and subsequently against driftors). They are often associated with fugitives, though ironically, runaway criminals are seldom found on-board drifts.