What happens when child entity updation fails or parent entity updation fails

I have a small  question here.  

Suppose I am updating a child entity or parent entity using C# code, and my update code is failed for parent or child. What will happen to other entity (i.e. child or parent). Will everything be roll-backed or we have to do it explicitly.

Thanks Regards

Prashanth

Thanks Regards Prashanth Kamasamudram Even the least work done for others awakens the power within; even thinking the least good of others gradually instills into the heart the strength of a lion.