A tool used in systematic desensitization, anxiety hierarchies are representations of situations that create anxiety. Typically these are images, videos, or other artificial representations of fearful stimuli that illustrate the people, actions, or situations that create the negative, phobic feelings.
Once these feelings (and their sources) can be visualized and recognized by the client they can be more easily dealt with. It is an increasing hierarchy with less stressful stimuli being presented initially and increasing in intensity until the client is dealing with the real deal.
An example of an anxiety hierarchy in systematic desensitization could be seen when treating a client for a debilitating fear of spiders. First the client might be shown a photograph of a spider at a distance. Once they are comfortable with viewing the image then they may move on to holding the picture. After this then perhaps they will watch some videos of spiders--once this is not stressful they may be presented with a fake rubber spider. After they are comfortable holding this fake spider then the behaviorist may move on to having the client in the same room as a real spider in a cage. once they are comfortable with this then they will move on to the spider being held by someone else outside of the cage. If the client moves up this anxiety hierarchy of increasingly fearful stimuli then eventually the client would hold the real spider themselves.