Colostrum
During the first days after the birth a woman produces special milk that looks thick, sticky and yellowish. This special milk is called colostrum and contains large quantities of antibodies and growth hormones. It enhances the development of the babys gastro-intestinal tract and its anti-infective agents act like a first immunization.
Colostrum has a laxative effect which helps the baby to pass meconium (the first stool) and it also helps to prevent neonatal jaundice by clearing the bilirubin from the gut. Although there is not much of it, colostrum is all your baby usually needs in the first days of life.
Transitional Milk
During the following two weeks, the milk increases in quantity and changes in appearance and composition. The immunoglobulins and protein content decrease whereas fat and sugar content increases. At this time, the breasts may feel full, hard and heavy. This normal engorgement of the mothers breasts sometimes referred to as the coming-in of the milk can be alleviated by frequent feeding.
Mature Milk
Mature milk looks thinner and more watery than cow's milk, which might sometimes be confusing. But it contains all the nutrients your baby needs for healthy development. Breastmilk is never too thin. The composition of mature milk changes over the course of a feed to suit the needs of the baby perfectly.
The milk that flows at the beginning of a feed is called foremilk. Foremilk is low in fat and high in lactose, sugar, protein, vitamin, minerals and water. As the feed goes on, the milk changes to hindmilk, which is richer in fat. Usually, the differences between foremilk and hindmilk are purely academic. There is no sudden change from foremilk to hindmilk. The fat content continues to increase from the beginning to the end of a feeding.
(from https://www.medela.ie/UK/breastfeeding/knowhow/composition.php)