A hormone secreted by the adenohypophysis (anterior pituitary gland), which is made up of 199 amino acids with a molecular weight of about 23,000 daltons. Prolactin has many effects, the most profound of which are to stimulate the mammary glands to produce milk (i.e. lactation).
Sometimes, newborn babies (males as well as females) secrete a milky substance from their nipples in a phenomenon called witch's milk. This is caused by baby being affected by prolactin circulating in the mother just before birth and usually resolves soon after birth.
There is a diurnal variation as well as a ovulatory cycle variation in prolactin levels. During pregnancy, prolactin levels rise as rising estrogen promotes prolactin release, causing further maturing of mammary glands, preparing them for lactation. After childbirth, prolactin levels fall as the internal stimulus for them is removed. Sucking by the baby on the nipple promotes further prolactin release, maintaining the ability to lactate. Usually, in the absence of galactorrhea, lactation will cease within one or two weeks of the end of demand breastfeeding. High prolactin levels also tend to suppress the ovulatory cycle by inhibiting both FSH and GnRH.
As a contraceptive, demand breastfeeding is said to be more than 90% effective in the first month of pregnancy even if no other forms of contraception are used, with decreasing effectiveness in successive months. Those are pretty good figures, though it is advised that this should not be relied on as the only contraceptive for any long period of time...
Too much prolactin can cause galactorrhea and disturbances in the ovulatory cycle and are often the result of a prolactinoma, a pituitary gland tumour.
source: http://http://www-medlib.med.utah.edu/kw/human_reprod/lectures/prolactin/