The cell is the fundamental building block of this model. It contains all of the state variables that describe that particular piece of the Earth's surface, as well as methods for interpreting those variables. A cell also has methods for setting and altering these variables, and for sending requests to other cells that they provide information or execute methods.
It is possible for a cell to possess other objects, including objects that encapsulate complete models. These objects may also respond to requests from other objects that they run, change parameters, etc. There is also a facility to have each cell contain variables for its future state; execution of each cells actions would finish with the cell transferring its future state to its present. By having this functionality unique to each cell, it's possible to have cells (or the models or components encapsulated in the cells) run and update themselves at their own rates, allowing a cell to operate at a temporal scale completely different from its neighbors, or to perform simultaneous updates in complete synchronicity with the rest of the cells in the landscape at the discretion of the modeler.
In essence, all the ``interesting'' (application specific) behavior of the model occurs at this level. The cell becomes the place where landscape models are handled. The importance of this role should become obvious as specific applications are discussed.