Apocrine glands are glands which produce sweat. Most modern mammals have apocrine glands and use them to communicate. Located in between the skin and subcutaneous fat, these glands are tiny spirals that secrete sweat into the hair follicles found around the aerolas, armpits and genitals. In addition to these there are specialized apocrine glands around the eyelids called Moll's glands.
The secretions themselves are milky, viscous and do not develop an odor until coming into contact with bacteria located on the skin's surface. It is likely that the vomeronasal organs can detect the scent of these secretions before the bacteria have a chance to do their thing. These secretions contain pheromones which allow for easier identification of the mammals in question—identification in terms of lover-lover, parent-child, etc.
Once when my sister was detoxing, she stank superbad and after I complained she blithely informed me that humans are biologically programmed to find the body odor of siblings abhorent in order to prevent incest. Said explanation was accompanied by her raised arms, much to my dismay.
Interestingly enough, apocrine glands are in fact merocrine and not apocrine. (They are merocrine because the secretions pass through an intermderiary tissue (ie hair) before leaving the body.) They were first thought to be apocrine and the name has stuck like bad BO.