The most common understanding of grief is called the "Kubler-Ross Model," from psychologist Elizabeth Kubler-Ross's 1969 book, On Death and Dying, which outlines 5 stages of grief:
Denial
Anger
Bargaining
Depression
Acceptance
Some more information about the Kubler-Ross model:
Psycom.net: https://www.psycom.net/stages-of-grief
Psych Central: https://psychcentral.com/lib/the-5-stages-of-loss-and-grief
This model has come under some criticism recently. Here are some alternative views:
This article argues, "It's Time To Let the Five Stages of Grief Die"
This article from the Australian Psychological Association describes some more recent theories and attempts to describe grief and bereavement in terms of greater complexity.
This site summarizes the Top 4 Criticisms of The Stages of Grief