Grief is the constellation of internal thoughts and feelings we have when someone we love dies. Think of grief as the container. It holds all of your thoughts, feelings, and images of your experience when you are bereaved. In other words, grief is the internal meaning given to the experience of loss.
Mourning is when you take the grief you have on the inside and express it outside of yourself. Another way of defining mourning is "grief gone public" or "the outward expression of grief." Talk about the person who died, crying, expressing your thoughts and feelings through art or music, or celebrating special anniversary dates that held meaning for the person who died are just a few examples of mourning.