A metabolic disorder is a medical disorder which affects the production of energy within individual human (or animal) cells. Most metabolic disorders are genetic, though a few are "acquired" as a result of diet, toxins, infections, etc. Genetic metabolic disorders are also known as inborn errors of metabolism.
In general, the genetic metabolic disorders are caused by genetic defects that result in missing or improperly constructed enzymes necessary for some step in the metabolic process of the cell.
The largest classes of metabolic disorders are:
(for further details, see inborn errors of metabolism)
A fourth class, the channelopathies (some of which cause periodic paralysis and/or malignant hyperthermia) could be considered to be metabolic disorders as well, though they are not always classified as such. These disorders affect the ion channels in the cell and organelle membranes, resulting in improper or inefficient transfer of ions through the membranes.
There are also a number of other metabolic disorders (such as myoadenylate deaminase deficiency) which do not cleanly fit into any of the above classifications.