When a child is born, everyone says, "it's a boy" or "it's a girl." Yes, our society assigning someone's gender by their biological factor.
The term transgender describes people whose gender identity differ the sex to which they were assigned and person's gender identity doesn't match with his/her physical sex.
They were born as male with feminine feelings. They feel they Were born in the wrong body. For this reason, some transgender People choose to have surgery to take physical form of their desired Sex.