These stages are quite famous because they are commonly used to describe those faced with loss.
The first stage is denial. This stage is unsurprisingly the stage where someone who is impacted denies that the thing causing them grief, and is marked with them avoiding proclamations of their grief.
The second stage is anger. By this stage the person has moved on from denial and is now angry that such a thing has happened. Their demeanor when involving something that remind them of their grief, will be that of anger and hostility.
The third stage is bargaining. Sometimes this is marked with wanting to take something back or blaming themselves and thinking about what they could have done to prevent such things that caused grief.
The next step is depression. By this stage the has pretty much come to grips with the event causing their grief, and knows there is likely nothing they can do, and so this feeling over powerless and hopelessness leads to general depressive tendencies.
Lastly Acceptance. This is marked as the period where the person has come to grips, and has begun to swing away from all other tendencies involving the even causing them grief. While they may still have general unhappiness when something remind them, they are not overly affected.