They are the daughters of the ruling class. They can be adopted if their parents somehow broke Gilead's laws, they can be naturally conceived by fertile Wives, or they can be conceived by Handmaids during the Ceremony. They wear pink (white, in the novel) until they're old enough to be included in one of the other classes.
The Class they're going to belong to depends on a number of factor, for example:
are they respectful of Gilead's laws?
are they fertile?
are they getting married?