

It has largely replaced Arrow on Node diagramming. PDMs represent activities as boxes that are assigned properties of the activities they represent.
See here for a good Article on benefits of PDMs. PDMs are popular and useful because they can:
- Help find the critical path
- Help define the amount of time required
- You can use them to crash projects
- You can use them to flatten resources
- Define dependencies/precedence
- Identify lead and lag times
The fundamentals of a PDM element are represented in the below diagram:
- Precedents or dependencies
- Earliest start
- Estimated duration
- Earliest finish
- A description of the activity
- Latest start
- Float time
- Latest finish
In practical terms revisiting the basics has enabled me to be more effective in plotting schedules into Project. I used to tackle tasks/activities in a sloppy fashion, but since walking through the basics I have had to put together two project schedules, and I took a more disciplined approach to each and have a much more satisfactory result, especially when the changes start rolling in.

Reference: www.meglan.com, http://www.pmicos.org/fse.asp, http://www.asapm.org/asapmag/articles/ps08_earl.pdf