What is the relationship between the hadsm and the work unit?

hadsm is the program, and a work unit is a set of parameters it uses to set it's starting position.
If you look at Project Stats to the left of here, you can see the latest 10 results returned, with a list of the parameters used for each.

There is more info under Climate Science, also to the left, especially <a href="http://www.climateprediction.net/science/strategy.php"> here.</a>