There are many hair loss conditions that are known and documented in medicine. Here are a few examples and an overview definition of each:
Alopecia Areata (AA): AA is a common disorder whereupon there are small bald circular patches found on the scalp.  More than likely the first incidence will occur before the age of 20. The patches of hair loss remain unresolved indefinitely or will resolve quickly without intervention. The cause is thought to be an autoimmune issue. The treatment is simply corticosteroid treatment or minoxidil. The worse case scenario would be Alopecia Totalis where the entire scalp is affected and can be aesthetically debilitating.
Anagen Effluvium (AE): AE is a type of hair loss caused by something affecting the hair cycle of growth. Anagen phase is the phase that 90% of scalp hairs are growing in and a drug such as chemo for cancer treatments could affect 90% of the hairs on the scalp. In AE the hair loss is temporary as soon as the chemo drug is stopped.
Cicatricial Alopecia (CCCA): Hair loss that involves scarring. An older term for this alopecia was hot-comb alopecia or follicular degeneration syndrome. CCCA affects African Americans and was originally thought to come from the use of hot oils however this definition has been changed. Discoid Lupus and lichen planopilaris account for 99% of cases. Transplantation into the areas of scarring are with hesitation because of yield issues, however more evidence has been pointing to having two transplants for resolution.  The first one would be to “fertilize” the area and the second transplants yield would be greater than the first.
Chronic Telogen Effluvium (CTE): A condition found in mostly women where upon there are serial bouts of shedding but never progressing to total baldness. In men Ferritin levels are checked and augmented along with using minoxidil. in women acute stress, trauma, hormonal changes, crash dieting, post partum, malignancies, hepatic failure, or even shock loss via hair restoration can cause CTE. Usually the CTE  is reversed when the conditions are treated etc.
Traction Alopecia (TA): A condition of hair loss brought on by constant “pulling” forces on the follicles. African hair types are seen with this condition when there are use of tight braids.  Other religious communities such as those who practice wearing turbins can cause traction alopecia. It is a permanent loss that can be treated with a hair restoration.