Legal Custody vs. Adoption

Does anyone know the risks of obtaining Legal Custody of a child versus adoption? What about obtaining legal custody while pursuing adoption? Can a parent give legal custody to a non-relative and then terminate his or her rights to lead to an adoption? A little background...we are in the process of possibly gaining custody of either one little girl or the little girl and her sister from people we know. I am just trying to know what my legal rights would be. This isn't an adoption from foster care as children's services is not involved. I am thinking obtaining legal custody might be the place to begin. Thanks in advance.