The human nail unit is a highly specialised, keratinised structure belonging to the group of skin appendages. It performs many important functions necessary to maintain homeostasis, such as protection of the distal phalanx against damages or stabilisation of the fingertips. Human nail unit is formed by the nail plate and the proximal nail fold, lateral nail folds, nail matrix, nail bed and hyponychium. Human nail unit diseases are often diagnosed with delay, mainly due to anatomical conditions, but also due to understating the problem not only by the patients themselves, but also by doctors. Changes within the human nail unit are a frequent component of many dermatoses or separate disease entities. The main disturbances associated with pathologies within the nail matrix include the Beau lines (transverse pits indicating temporary disruption of the nail plate growth), onychomadesis (nail shedding), onychorrhexis, nail pitting, koilonychia (spoon nails) or trachonychia (sandpapered nails). The group of diseases associated with disorders of the nail bed includes onycholysis (loss of communication between the nail plate and the nail bed), subungual hyperkeratosis, apparent leukonychia (associated with the swelling of the nail bed) and subungual petechiae. The article also presents selected nail diseases associated with the deposition of dye (including melanonychia), some congenital disorders of the nail unit (such as, for example, malalignment of the great toenail or nail-patella syndrome) and selected manifestations of systemic diseases within the human nail unit. Getting familiar with the correct onychological nomenclature and presented issues will certainly facilitate establishing a dialogue between specialists in skin diseases and general practitioners.