Adoption is the act of taking up the legal responsibilities as a parent(s) of a child that is not biologically yours. It requires taking someone as your own and being able to take care of that person. The population group that is usually available for adoption are children.
Adoption requires a legal process which is dependent on your geographical location in the world. It could be done through an agency, internationally, independently, and so on.
It is true that a lot of times, parents adopt a child when they do not have a biological child that they can call their own. However, a point often overlooked is that child adoption is not only for parents who cannot have their own biological child. It is also for parents who already have a child and are financially secure. Furthermore, it is for parents who feel the need to help another child have a family and a better life.
For this reason, parents who are financially capable are encouraged to adopt a child and give him or her, an opportunity to have a family and a brighter future.