Hygroscopic water is present on the surface of soil colloidal particles and is held very strongly by the attractive forces. It does not exist in liquid form but in the vapour form. It is held so tightly by particles that it is not present for absorption by the roots of the plants.
Capillary water is present in the pores of the particles and are held tight enough so gravity cannot pull it. It exists in liquid form and is available for absorption by the roots of plants.
Final answer:
Hygroscopic water: The fine soil particles imbibe water and hold it very tightly.
Capillary water: The water that remains in the soil after gravitational water is drained out.
Hygroscopic water is not available to plants as it is held so tightly by particles.