I think it would help a lot if you added what the circumstances were. Did their other parent die? Are they in prison? Was conception done through a fertility/sperm clinic? The product of an affair? If it is a man, Is he aware he has a child? If it's a woman is she unsure of who the father is? Was there a failed relationship or divorce?
There are many different scenarios which would elicit different ways to approach the subject. I believe it is important to let the child come to you with questions instead of bringing up things they aren't ready to discuss or even have concerns about. Once that happens I would explain that there are many types of families, some with two parents, some with one parent, some where kids are being raised by grandparents or other family members, adoptive parents, and so forth. However the only thing that matters is they are loved and supported. The primary responsibility for a parent is to love and help a child build his or her self-esteem and prepare them to become a productive self-reliant adult.