Crying (also known as sobbing, wailing, weeping, bawling, or blubbering) is shedding tears as a response to an emotional state in humans, says Wikipedia. The act of crying has been defined as "a complex secretomotor phenomenon characterized by the shedding of tears from the lacrimal apparatus, without any irritation of the ocular structures."
The medical term for this is to lacrimate, which also refers to non-emotional shedding of "tears". A neuronal connection between the lacrimal gland (tear duct) and the areas of the human brain involved with emotion was established. No other animals are thought to produce tears in response to emotional states, although this is disputed by some scientists.
In many cultures, Wikipedia goes on, it is more socially acceptable for women and children to cry, and less socially acceptable for men to cry. Tears produced during emotional crying have a chemical composition which differs from other types of tears. They contain significantly greater quantities of the hormones prolactin, adrenocorticotropic hormone, Leu-enkephalin and the elements potassium and manganese.