Leakage current is the current that flows from either AC or DC circuit in equipment to the chassis, or to the ground, and can be either from the input or the output.  If the equipment is not properly grounded, the current flow through other paths such as the human body.
The leakage in devices is largely due to the imperfections in the insulators or materials that make the component such as the semiconductors and capacitors.  These results into small current leaking or flowing through the through the dielectric, in the case of a capacitor.