Hijra's are "the third sex", intersex or hermaphrodites, also gay or transexual/transvestite.
Hijra's are a caste to themselves, you see them begging on trains or roadsides, or in some areas working as prostitutes. They also go to weddings and births, dance, cause a racket and are paid to leave, traditionaly they have the power to bless or curse people, so if your nice to them they will bless the wedding/child otherwise a curse. They have a strange role in society - not accepted, but still a part of daily life.
I am no expert on the Hijra but I find them absolutely fascinating, rejected by their families & mainstream India, but instead of dying quietly they fight on and create their own society. Incredibly strong, inspirational people.
I think people throw around the word "caste" a lot without understanding what it means. And it just adds to the ignorance of what a caste system is and how is operated differently in different parts of India. Hijras are not a caste, they're a community. All castes are communities, but all communities are not castes.